I've written about my own similar problem as the op. Researching it finds it happens often. It took 3 days for my son to receive a pic I sent, though I thought he had received it when sent.
This is, from my understanding, an issue with Apple treating all messages as mms, and some deep state conspiracy to prevent me from doing the above. JK. The Android to Apple and vv messaging is not as dependable as Apple to Apple or Android to Android.
We worked it out by both having the Viber app. Now there are quite a few more apps to solve this.
This too:
https://www.imobie.com/support/iphon...om-android.htm
Good luck