Thank you’s not going through

Hi, I’m sending thank you notes but many of them are not going through. After sending, the donation still says “Send a thank you.”  The successful ones have a check mark and say “Thank again”. 
I’m not doing anything differently. Always using iPad or iPhone, but it looks the same on both. 
One donor who I thanked said they didn’t receive the thank you email. 
I logged out and back in again, no change. I’m the beneficiary and I’ve been able to do the thank yous many times before.