Why is it that everything that I have ever purchased to mount on my bike NEVER fits? The mount for my U-lock. Nope. Rear headlight is supposed to clamp on the seatpost, but it doesnt. Bike computer hard mount? not a chance. Cheap rear fender? not even close.
Im going through zip ties like nothing and I dont trust those to stay on half the time. Does anyone have any other solutions?

Are the clamps too small to fit around the tubes or are they too big and won't compress far enough?

If too big, wrap sections of an old inner tube to make a shim.

The clamps are always too small

I'm guessing you have extremely oversized tubing on the frame and forks. But that doesn't explain the seat post issue Sizes there are pretty standard and so are handlebars. What's the bike brand and model?

There are few standards in the industry and they change for marketing purposes.

For my Ulock mount, its the top tube of my Trek 7000 (POS commuter), the seat post issue was on a Fuji Roubaix 2.0 (my sons), also have a Felt F75 that I havent had to zip tie anything on it yet.