How exactly do you have it set up? A 55 watt should be fine for a 24x24x16 cage.
Are you using a temp gun to check the floor? The top of the hide?
Snakes dont need belly heat if they got warm enough ambient on the hot side. You really have 2 choices. I use the cooler ambient temps with a single hot spot via belly heat. The other method is heat the whole cage and just give them a warm spot. The RHP will do that if you use something like a hide under it or a piece of slate or flagstone, anything the RHP can heat up under itself which in turn will radiate heat off the item to warm the air.
Also try to plug the RHP straight into the wall and bypass the t-stat to see if it heats up more. Try a different outlet too. These are all the things Bob at Pro Products told me can affect a RHP. Also put something under the cage to insulate like a piece of plywood.
And I'm not really sure where the 90F hot spot idea came from but none of my snakes like it that hot. I give them a hot spot of 86-88F and the rest of the cage sits around 77-80F depending on the room temp. Most of the time they spend on the cool side or just stretched out in the cage.
But anyways, first thing i would do would be plug the RHP straight in the wall, shove a hide under it and let it run full power for an hour. Then temp gun the top of the hide and look at your temp gauges and see what the ambient temp is on the cool side. Ideally, i would shoot for a 86F or so temp around the RHP with a 78F or so temp on the cool end of the cage. Can be work since you are using a fairly short cage so you dont have much room for a gradient.