Was the number belonging to the SIM in your current phone previously used in an Apple phone?
You say that it has stopped receiving messages from her phone so it must have worked in the past, therefore it probably isn't this, but..
If it was ever used in an iPhone, say one day you swapped SIMs and put your SIM in her phone: it's worth checking on the Apple site that the (your current) phone number is not registered for iMessage.
When you move from Apple to something else you have to deregister your number otherwise texts sent from Apple phones won't reach you.