Assuming you've checked other fixtures in the bathroom or kitchen or whatever, and all is well, there are a couple of potential causes of this.
During the install, if you 'kinked' the copper feeds, the flow will be inhibited. Inspect the install. Second, check that the line valve is turned all the way on. If it is, and the flow is still poor, disconned the line and try each line into a bucket. If you can screw on a flexible extension to the shut-off, and run it into a bucket, you should have your answer. If the flow is normal at the shut-off, the problem is between it and the faucet. Maybe the line between the shut-off and the copper feed line of the fixture is clogged. Maybe the copper line, itself, on the fixture is kinked or clogged. I'd do this with the screen in place if you can be sure it's NOT the problem.
My best to ya and hope this helps.
PS: The Plumbing Forum is a great site. Be sure to thank them there too if you get your answer. You know a plumber's rate. THeir free advice is well worth a thanks. ANd if you do all the checking I've suggested, and the problem still isn't solved, be sure to tell them what all it was that you tried.