BB Lockring won't completely tighten I am trying to put the lockring back on my BB after adjusting it to reduce the play, but I am unable to full tighten it. I can hand tighten the lockring, but as soon as I try to use my hook spanner to tighten it more, it kind of slips and becomes loose again.

Are my threads toast? Is my lockring warped? Or could it be something else?

I did this same maintenance on my bike less than two weeks ago and I didn't encounter any problems.

FYI: My bike is a 1980's Raleigh Capri 410 with regular threads (righty tighty, lefty loosey)
I had a similar issue last year. The threads on the frame to take the bottom bracket were too worn to hold the bottom bracket in place. A bike mechanic should be able to confirm this for you (for no fee). After some research I found two solutions, one of which is considerably cheaper than the other:

1: bike frame specialists / frame refurbishers can re cast the frame thread with a tough resin, and then rethread the resin, allowing a bottom bracket to sit securely. This is the expensive option.

2:You can buy bottom brackets that tighten onto them selves to stay in place. This is a sample: http://www.kingstreetcyclery.com.au/self-tightening-bottom-bracket-u118.html
However! do your research before you buy; there are different spindle lengths and other options. I replaced mine with one from ebay for about £40. After retightening a few times initially, I have had no problems since.