I've been away from the board for a few years, but I have to drop in to comment on this thread as it's something that has frustrated me over the years.
Poly does not give you more spin. That has been proven in tests with a machine that hits the ball the same way with the same swing trajectory, with different strings. The result was no, repeat NO, increase in spin with poly over gut or nylon.
So why does poly increase spin?
Monster touched on the answer, but didn't give it. It does have to do with the lack of trampoline effect with poly. It does give less distance. The fact is, I don't think I've ever read anybody actually give the answer.
The answer is the less distance you get from poly means you HAVE to swing harder to get the ball to go the same distance. You need to increase the velocity of the racquet as it meets the ball. With the natural upper cut swings we tennis players use, that gives us more spin. More spin however also means less length. So we have to swing even harder, to overcome the lack of length and the spin, giving us even more spin.
Eventually, we find the balance between speed of swing, height trajectory, and depth.
So there is nothing wrong with the poly you are using, and there is nothing wrong with you. You just need to swing harder (correctly, to keep from injuring yourself), and add height to your ball to get it to go over the net.
It's a matter of practice and repetition (which is how you learn tennis in the first place). If you give yourself a chance, you will get used to it. I didn't like it at all when I first hit with it as well. I got used to it.
The bottom line is; the faster racquet speed from the harder swings gives us more spin, though this is necessary because we are hitting with poly.
The same effect can be had by stringing your racquet at 80 PSI (which is what Bjorn Borg did). That will give you the same lack of power that poly gives you. It's just not recommended as you will have racquets snapping at odd times like Borg did. Manufacturers won't replace a racquet that breaks because you strung it outside of their recommended parameters.
One final comment, if you don't swing correctly, you should not be using poly. You should be swing with your shoulders not your arms. Talk to a tennis pro if you are not sure about it.