Doesn't sound like the way an Invincible should be acting. When it's clean, there shouldn't be any smoke to speak of and about a half pot of pellets should be enough to satisfy the exhaust sensor and get the feed going. I'll describe what I've found in the stove assuming that you already know your vent piping is clear.
The air flow passages aren't really all that complex. Normally the inlet wants to be the one at the back of the auger assembly that feeds the air through the bottom of the pot and up through the holes. This passage does want to plug with cinders and pellet dust sometimes. If you haven't cleaned here it's accessible by loosening the thumbscrews for the plate on the front of the burn pot. The same air inlet allows some air to come through the auger itself. That should be obvious as the fire will actually burn down to and sometimes into the auger, but the flame will follow the air flow out and up into the firebox.
Once the air has made its way into the firebox, it's going to want to go up and over the baffles on both sides to make its way through the heat exchanger tubes. If both sides are clear, you should see a pretty symmetrical flame in the pot. If it is burning on one side or the other or the flame seems to be burning toward one side only, it's a fair bet the other side is plugging. If you haven't taken the baffles off, they are again pretty easy to remove with thumbscrews. There's a little bit of a secret combination to rotating them out of the firebox, but with repeated attempts and light drinking you will prevail. If it's been a while, there could be a pretty thick coating of ash and soot on the back of the baffle and the wall of the firebox.
From the top of the baffle, the air flows down to the bottom again and makes it's way into the bottom of the heat exchanger. This is essentially a U-tube that causes the exhaust air to flow up one side and back down the other to make it's way into the final passages going to the exhaust blower. The inside of these tubes can plug pretty easily and is the most likely place the air would be impeded since they are so hard to access and fully clean. There is a formed metal plate that is removable at the bottom of the wall that will give you access to the bottom of both tubes.
I have a small mirror on a telescoping handle that I can hold in there to reflect a flashlight up into the tubes. I would suggest you get one of these (auto parts places, hardware stores, etc.) as two years ago one of my tubes had the dried remains of a bat stuck in it and I would never have figured it out without seeing it. They are actually square tubes and if they are clean, you should be able to make out the straight walls all the way up on each. Normally when they are dirty, they will have formed a kind of round hole up the center.
I haven't found any drop dead - sure fire - only one to use tool to clean these out. You have to have something pretty flexible to get in the bottom hole and still straighten out enought to rod out the tubes. You can bend a piece of wire or stiff cable and just keep shoving up and down until you manage to break all of it loose. Keep the shop vac handy to get the piles out the way.
With the metal plates out of the way you should also be able to shine a light a little ways back toward the exhaust blower to see what these horizontal passages look like. Here again if there is ash and soot built up, you may simply be able to scrape it loose and vacuum it back from the firebox.
If the inlet, the heat exchanger and the passages to the blower are clear, then the only remaining spot is the blower itself. Get a gasket kit before you take this out as the blower gasket is a little sensitive to begin with and gets pretty brittle with age and heat. Bunch of small sheet metal screws holding a flange down. Once these are out, the blower motor and impeller come straight up. The impeller is metal, but fairly thin so be careful not to bend the vanes. Cleaning this should be pretty obvious, although tedious. With the blower out, you can also get another angle on the passages going back toward the firebox. While you are in the back looking at the blower area, you should be able to check the little gravity door on the air inlet to make sure it swings open ok.
With the blower out, you'll see that there is precious little opportunity for plugging after the blower before it gets to the vent. You can push something through it anyway just to ensure it's clear, but go gently remembering where you put that new exhaust temperature probe.
So on my stove that's it. If you have an outside air kit, you may have some more inlet piping to check, I don't.
Hope something in all that rambling helps.
----------------------
Wow, I just realized you have an insert and I'm talking about a freestanding. Sorry for any confusion and I hope some of this still helps.
Here's a link to a previous post I found from 2005. The guy with the reply is evidently still on the forum, so you might try him with a private message or e-mail and see if he can walk you through getting the board set. My feed rate adjustment is set at about 3.5 which is right around what Harman suggests as a starting point. This works pretty well for me with the ash ending up being around 1 to 1.5 inches at the top of the pot.