What other rules you have? Especially in the rule that sets the availability, and in the default rule.

How often does this approximately happen? If it happens quite often, you could try the newest beta as I've slightly changed the logic of availability restoring in it:

http://www.kolumbus.fi/~ks2877/profi...eta2_armel.deb

For beta1 users, the newer beta has some availability fixes.