Devices not discovering / Devices going missing

I am currently using the free version of OpManger. I have Discovered the devices I need to. A couple of them are still being put in to the Desktop Category despite being 2003 boxes. I have verified all snmp settings are on and correct.

The next problem is, domain controllers keep vanishing. I have checked all the default categories and they are no where to be found! The devices are still up and running. I have tried rediscovering the devices that go missing but OpManager says I have already used up my 10 devices. I realise this is because I am using the free version, but If i cannot find the devices, I cannot delete them.

Help!

Thanks