Hi,
If you have a little dirt on your car when you wax it, the wax will act really good at keeping the dirt UNDER the wax. Same thing happens on Rain X.
I would suggest cleaning with Windex - something with ammonia in it that will effectively remove the Rain Ex. The once it is clean, apply the Rain X and it will be much easier to clean the next time. Clean the inside, and then look at it towards a bright light, and see if there are any streaks left.
After the windshield is about 35,000 miles old, it can get a pitted look to it. It is actually microchips from gravel being thrown to the windshield.. You really can not get that out. I had to replace one half of my windshield once, and you really could tell how much cleaner and clear the new half was. A close look at the old one found a lot of microchips.
Fred.