Text messages not always received

There is someone that I text with every day usually several times a day. Occasionally a message I send to him he doesn’t receive. I look at my message history and it shows up on my phone, and iPhone 12 promax, but doesn’t appear on this phone, same phone. This is a two-way street is the same thing happens occasionally with messages he sends to me.

he is my live and caregiver so it is pretty important that we have reliable communication. Earlier I texted him asking him to stop at my pharmacy to pick up important prescription. It shows up as sent on my phone but he never received it. I sure hope someone can solve this problem for me! Thanks!