Ever decreasing license availability

I have now run into this situation twice and don't know what is causing it.

We have a number of license servers dotted around the country with varying numbers of licenses on each. On the latest problem server, 10 licenses are held and it has been distributing them quite happily but I have just had a call from a user who can't get a license as there are none available. This shouldn't be an issue as there are currently more licenses than users. Having looked at the log file, their server has only issued 3 licenses before disallowing any more users. Having looked back at the log file, this has been going on for some time but unnoticed as we have redundant servers that users can fall back on in the event of a problem - it just so happens that I now have more users running licenses from the redundant server than the proper one. This has happened before and resulted in having to re-start the license server to get everything back on track. I've just restarted this one and again, everything is back to normal.

Why are the numbers dwindling like this? It's not down to borrowing as this facility has been disabled on all our servers.