Text messages not being received by certain people

So my parents aren't receiving my text messages.  I was receiving two at a time from them and I had them reboot their phones and now I receive messages from them as normal but they don't receive my replies.  They have their own cell numbers on Verizon but share an Apple ID (they are iPhone 6 and 7 I think) and I am iPhone X.  Seemingly they are able to text everyone else as normal and so am I.  We have checked all settings and rebooted our phones.  What else might be causing the problem?  

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