Yes, when shoppers do not follow instructions and do what they are supposed to do, a company (ANY company, not just specifically Sinclair) has the right and often the obligation to reject the shop. Instructions include when, where, what you are to do, how quickly you need to report your findings and submit any receipts or other materials. I have had minimal questions about reports done for Sinclair and they have always been paid.
That being said, Sinclair's shops tend to be in the $10-15 fee range. This means that for you to make $400-500 per month, you would need to do somewhere between 25 and 50 shops per month for them. I don't know about the San Antonio market, but I suspect it is not dissimilar from my market, where there are generally 10-15 jobs posted per month and snagged off the board very quickly by other shoppers unless they are the real 'dog' shops. I just deposited a check today for March work done for Sinclair and it was under $75 including reimbursements for 5 shops. Many of their jobs require a financial outlay to be reimbursed and with some of their shops the reimbursement is more than the fee while in others, there is little if anything that can be purchased for as little as the reimbursement amounts to.
It is for these reasons that most folks who undertake mystery shopping do not limit their sights to only one or two companies but are taking what is interesting and available from any of the 200+ mystery shopping companies out there that may currently have shops in their area. For me, that generally means working with 30-40 companies per year and roughly half of what I "make" is in the form of useful reimbursements that either enhance lifestyle or substitute for needing to use household budget to make needed purchases. So groceries, gas, oil changes, dinners out and such are frequent 'shops' for me.