Iphone will not airprint

I have always been able to airprint from my iphones to my Hp printer. I am no longer able to do this. My computers still print to the printer just fine. I have tried every remedy I have found online and none solve the problem. It seems as though the issue has to be with the iphones, since they are the only devices with a problem. I have tried disconnecting and reconnecting the printer, the router, restarting the iphones all to no avail. Frustrated beyond belief.