Case trimmer cutting groove inside my necks!

[Jack]

Ol Joe's given you some good advice about checking the pilot for a burr.
If the pilot proves to be smooth....

......."After I neck size my once fired brass, I'm having trouble with my trimmer guide fitting too tightly into my case...."
Try trimming before sizing- the case neck will be expanded from firing, so your trimmer pilot should fit easily.
Another trick you can try is to put a dab of lubricant on the trimmer pilot- that can smooth things up a surprising amount. Imperial Sizing Die Wax works great- just a tiny dab every tenth case or so- and I'm sure most other case lubes would work, too.