Spare tire left on the road behind me

[BC10]

I've been messing with the stock OEM spare tire brackets the past few weeks. Issue that I run into is that once I think I have secured my spare firmly under the bed, I then drive off, but the spare tire eventually pops out and ends up on the road.

I've tried switching the holes that the bolts are in, with no success. Maybe my spare is to wide? But I am able to latch it on under the bed...

Some info:
The spare is on 15" rallyes, tire size 265/75/15.
I have a 68 Short bed Stepside C10

Any input is much appreciated. Thanks!