I have also been having problems with the SOTA spotting app sms gateway. I have now got it working for me, but I can’t say exactly what it is I am now doing right. One thing that might have an effect is the way the SOTA spotting app remembers all the field data when you check the box. I found, thanks to Marco, that the frequency data was going through blank, even though it said 7.031 MHz in the app. This caused the spot to be rejected. When I deleted the field and retyped the data fresh, the spot went through. That said, it now goes through all the time regardless of whether I retype or not, but it could be a bug the SOTA spotting app. Just something to try, I am no software expert.
73 de OE6FEG/M0FEU