Are you watching the time carefully before peeling? You can get considerable change in contrast and print density by over- or underdeveloping with Polaroid B&W materials; the stuff I'm most familiar with, 667, gets darker and more contrasty if I let it go longer, while it'll be light and "muddy" (and also prone to peel marks) if I peel it early. Temperature gets into this, too -- if it wasn't in the packaging, there's information in the data sheets (downloadable from Polaroid's web site) on how to compensate development time for temperature.
Bottom line, if you're doing everything the same way, you should get prints about as consistent as what you could produce in a darkroom with an enlarger. If you're not getting that consistency, especially within the same pack of film, you're introducing variations. "Bad batches" of Polaroid films usually come out with streaks or blotches, and stay light (or, with color, have color shifts) due to aging of the processing gel; I've got some Type 52 that expired almost 20 years ago that required 8x the normal process time to give even light, low-contrast prints (fortunately, it was almost free; the 52 I got with it was worth the shipping I paid even if I have to peel the packets in the darkroom and process the negatives in trays).