[Serusers] registered UA's

Hi All I hope someone can help me figure this out. We've had a ser server running for quite some time now on a Dell 2850 with dual cpu's. The load is very low due to the use of multiple mediaproxy servers to handle the RTP streams. We have about 2300 UID in the mysql database but at any given time only about 1300 show as registered in the location table and we have customers complaining about not receiving calls. When tech support has a customer power cycle their ATA, it registers just fine and they start receiving calls. What could cause this? From the beginning, I noticed this was happening but didn't give it much thought as the number of UIDs was low. But now it is a growing problem. SER version is 0.9.0 Any hints or help would be appreciated. Rick