richardh
Might be this:
IOS Contacts app can define a contact's cell phone as an iPhone or as a regular mobile/cellular phone. Make sure you are listed in their Contacts as the latter, or else Apple will try to send you an iMessage, which you cannot receive. This is especially likely to occur when switching to an Android from an iPhone.
Also, using anything other thsn the stock messaging app has sometimes caused problems, especially outside the Admin profile. And, you might need to make sure inside that app that Settings > Advanced > Send a single MMS to all recipients is selected.