I have only had payment issues with one company, and that's Mystique. On one occasion, I waited 6 months for payment. In my experience with Mystique (for which I no longer do any work), the only solution is to repeatedly email John. He apologizes, he makes one or two out of four or five payments, then stops responding. Then further emails lead to further apologies and more payments. Eventually, you'll get your money. Or at least I eventually got mine. But in the end, I probably put as much effort and more time in the follow-up than I did in the shop visits and reports themselves. I am still owed some bonus money from shops I completed in March, but I have written that off... just as I have written off Mystique Shopper.
There are so many threads about their poor payment performance that I don't think our situations are isolated. This is just a theory, but it seemed like John would respond only when there were new jobs posted on Mystique's job board. Then for months, while there were no jobs posted, I wouldn't hear back from him. Then, suddenly he would respond just when new jobs were posted. Perhaps they either have a cash flow problem at Mystique. Or maybe so many shoppers have dropped them, he needs to redeem himself each time he has visits to schedule...