Open SMS API misses 6th and 7th message.

I am using the following call to send SMS message, and have been sending about 10 messages per day since May 2020.

https://joinjoaomgcd.appspot.com/_ah/api/messaging/v1/sendPush?smsnumber=01234&smstext=Hello World&deviceId=xxxxxxx&apikey=xxxxxx

It always missess the 6th and 7th message ??, and I end up having to submit those ones again, any idea why ?

Regards
Arran .