Raising the Hellcat's landing gear

I've had the Hellcat awhile, but I've never flown it much because of a problem
with getting the landing gear to come up after takeoff. The gear will come up
maybe 1 of 3 takeoffs; the rest of the time it stays down.

I assume it's something I'm doing or not doing in the warm up procedure, but
for the life of me I can't figure out what, since whether the gear goes up or not
seems to be random.

I'm taking off with the engine running, and I usually wait awhile, with the engine
running, and watching the temps of oil and CHT, but it doesn't seem to make
much of a difference in the end. And I`ve tried with and without different payloads.

Any suggestions appreciated, since whenever the gear does come up, it`s a great
plane to fly. Thanks - Bob