Router Problem...

Hi,

I recently purchased a used router and bit kit (around 20 bits) in a bundle of used power tools. When I bought it, it had already had a straight bit locked in it. When I removed the straight bit and installed a roundover bit, I found the collet wouldn't tighten enough to secure the bit from sliding out. I immediately assumed it was an issue with the bits, their use, and their resulting shank diameter (unfortunately I cant confirm this as I don't own a micrometer).

I went over to my local department store to get some new roundover bits and before I bought them, the representative let me see if they would fit. However the same issue occurred and the bit just slid out even when fully tightened.

For reference, all of the other bits in my kit all fit fine and tighten fully.

Anyone know what's going on? I'm quite confused and can't seem to find anyone else having a similar problem.