OK, so what's happening is the replay when clicked only once has to fully go through until you can slo mo. So basically, say you have a 300 frame replay, you'd have to wait for those 300 frames to go alllllll the way through and then you'd be able to slo mo. If you just press "Quick load" as well, that doesn't let you do slo mo, only if you wait. If you click "Load", then you will be ablle to get slo mo.
Sorry about the explanation, I suck at explanations.