Q: FaceTime is not allowing me to call certain people, others don't show for FaceTime option??

I recently purchased an iPad, and I already have an iPhone, using separate apple ids, I'm unable to FaceTime the iPad from the iPhone but I can FaceTime any of my other contacts, with the exception of an iPhone... my friend can FaceTime my iPad from her iPhone but i can't FaceTime my own iPad from my iPhone or a couple other people from my contacts, but some i can. what is going on??? help me, please. i have tried toggling feature on and off as well as restarting and resetting device and its still doing the same thing, the little FaceTime camera isn't even showing for one of my contacts or for the contact of my iPad. both devices are connected to wifi and able to use cellular data, features are turned on and signed in and there are no restrictions. is there anything else I can try?