It's understandable to barred if any shops weren't completed per instructions. Over the years I've found that if you have a valid reason you cannot fulfill or complete an obligation, many times providing the contracting source this information will allow you to make it up or take a leave of absence.
Lindsay, from my experience so far, I've never seen any MSPs suspend a shopper's access to their shops just for being an inactive shopper - their reasons mostly include stuff like "too many cancellations", or "poor work quality" or when a shopper gets very unprofessional and argumentative.
If some MSPs have suspended your access to their shops for whatever reason, you can always reach out to them, ask them to let you know what the exact reason was, and see if you can work with them on resolving it and getting a second chance.
There is one MSP (MarketForce) that will deactivate you as a shopper if you do not complete at least one shop every three months. I have been deactivated for this very reason twice - and have had an exchange with their help desk about.
For those not familiar with this company, it is likely the single largest MSC and does not use the iSS platform.