Help with XRS-50 home gym

Hi everyone, hoping someone here can help. I have the Gold’s Gym XRS-50 home gym. Bought it a few years ago, used it, then stopped for a couple of years to recover from a fractured C1 vertebrae. I’m getting back into it very slowly but I’m having an issue.

On some of my arm exercises I’m only using the top weight and it’s driving me nuts because 9 times out of 10 the bar pops out of the bottom weights and get’s stuck. So then I have to stop, put the bar back down into the slot and start again. It only seems to happen when I use just the top weight, not if I use more than that with the bolt. Any ideas on what’s going on?

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