This problem I locked a slots coin in my inventory but everytime I get another one locked one becomes unlocked

That's odd. Check the bug report forums and see if it's been reported yet. If not, leave a detailed report, including what other items you have, how many items were locked, what browser you're using, etc.

How many do you already have locked? You can only have five items locked at one time. Anymore than that and it won't work.

I only have 4 items locked at one time

Then I suggest make a ticket, as Fyre mentioned.