I do quite a bit through iSS. Every company has a different pay schedule. What I would do is look at the independent contractor agreement for the particular MSP for which you did the shop (it's easily found on the iSS site). Then, look at the job in your history - the date you performed the shop will be clearly listed. You can then compare the date with the the agreement and confirm that a payment is missing. You can then contact the MSP directly - contact information is provided.
I've done a lot of shops through iSS and, so far, every payment has been right on time. Some pay on day 45 after the shop is approved. Some pay on day 60. Some pay on the 15th of the month, others on the 20th or 30th. I keep a spread sheet list of all my shops and one of the columns is "expected pay date," which I fill in when I submit a shop.
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