If I have done 25 shops and have been paid for three, it would not be worthwhile for me, even for fun. I would definitely check why I am not paid for all of it.
However, this is mystery shopping and we are independent contractors. Some of the companies, and it is just August, are slow payers. Purple portal pays twice a month and their August check was supposedly August 10th. Still early and you are indeed doing well if you have received three payments already for doing August shops.
Yes, I agree it ia worthwhile not only as a second job but even as a full-time business, as attested to by many in the Forum. And there are better paying shops, if you look around and are committed enough. JMHO. Sign up with as many companies and you will find them. Good luck!