A couple of things come to mind, only because I dealt with this a while back and it ended up being the last thing I would have thought of. It could very well be worn out plates, depending on mileage and riding style. That's easy enough to check, just take it apart and inspect the plates. Do you have any oil leaks, particularly around the clutch? Oil dripping onto the plates can also cause slipping. Also, any sort of decreased pressure on the plates can lead to slipping, so make sure all the springs are there and snugged down. This one is less likely as there are a fair amount of folks riding with as few as three springs and report no slipping.

Does it slip when it's cold, or only when the bike warms up. The reason that I ask is that if you have any air in the clutch line, as the bike warms up the air will expand and have the effect of pulling in the clutch lever slightly relieving pressure on the plates and causing slippage. If you rebuilt the slave and you're only having slippage when the bike is warmed up, chances are you have air in the line. Try bleeding the line and see if it goes away. One thing that can make the process much easier is to install a banjo bleeder up on the master cylinder. They don't come with one from the factory and air tends to collect at the connection between the master cylinder and the clutch line due to the fact that it's the high point in the system. If you throw a banjo on there you can bleed it from the high point, lessening the chance that you'll get an air bubble.

I went through all this after a left hand low side which hit the left bar so hard it broke the clutch lever clean off around my hand. When I got it all put back together, I replaced the lever and I kept having slippage when the bike got warm. I replaced things one by one, starting with the least expensive part first and working my way up. After replacing everything but the master cylinder, I finally broke down and asked the last guy in town (a mean SOB of an old race mechanic who I avoided whenever possible). I happened to catch him on a good day and he very kindly explained to me that the Brembo MC's have valves in them that if the lever is ever compressed incredibly rapidly (as in a low side) these valves essentially get flipped around backwards and each time you squeeze the lever thereafter it sucks air into the line. Unless you've done the same, I think you're problem likely lies in the rebuilt slave and you probably didn't get all the air out. So my MC explanation was probably a waste of your time, but maybe someone else will have this problem some day and save themselves from replacing their entire clutch in the future.