Needing to cover part of the feed is totally normal guys, don't sweat it too much. I ended up choking down my feed quite a bit, it's now roughly 5"x5". That allows me to run it open and it almost never creeps, but depending on the fuel, there are still times I need to cover it.
Fuel is certainly the most important factor in whether the fire creeps up the feed. Too wet internally to burn down and fall in before the flame front climbs the wood is one way it can manifest. Wood that has a lot of very dry surface area and not much fuel value can also just plain overload the system and creep can happen that way. I've noticed a huge difference in burning fir vs. maple vs. alder, etc. They all behave differently, and moisture content and split size, in the round, etc. all make a big difference. Sticks are one of the worst offenders for me, unsplit branches and such. They just stay wet inside for a very long time, but usually have really flammable outer layers.