Fork oil drain plug won’t tighten.

[Slofun]

I changed the fork oil today in my RPS Hawk 250. I raised the bike so the front wheel was off the ground. I removed the 6mm cap screws at the bottom of the fork and drained the oil. They were very tight. Then I removed the 17mm caps at the top of the fork tube. I then replaced the lower drain bolts but noticed that even though they threaded in ok, they wouldn’t tighten up. So I added the new oil anyway and replaced the top caps.

What did I do wrong? Why won’t the lower cap screws tighten up? Any way to fix this?

Thanks a lot for any help you can give me!!