Dynamic Groups doesn't update automatically

Hello, I've got a problem with my dynamic groups.

My dynamic group has the preset to get any host that starts with "u1-not" and add the host to it.

Is there anyway for it to update by himself, and, add the host automatically to its cache?

And that's not all yet. I can, manually, add a host using the replay button, but, when I change the host, merge the duplicat on the merge duplicates option, it doesn't leave the group when I use the replay button, even if it doesn't match the criteria that I mencioned earlier.

Thanks for helping.