I've maxed out my SDP this deployment for the first time. If you read all the official mumbo jumbo on the SDP site, it says they only update the balance on your SDP in myPay on the first of the month. So if you put money in on the second of the month, it won't show up in your balance until almost a month later. Also, any money you put in after the 10th won't begin to accrue interest until the first of the next month.
Apparently DFAS doesn't have computers that can transer money like every bank in the industrialized world, so they rely upon carrier pigeons to deliver your money from the AOR to CONUS. If the pigeons get confused and try to fly west, the jet stream can delay your deposit indefinitely.