A number possibilities:
1. A lot of mini-labs make lousy prints. Between old chemistry and operators who don't care, the results can be awful. Bring your negatives to another lab and bring one of your bad prints. Ask if they can do better. You may have to shop around to find a lab (and even an individual who works at the lab) who gives a darn. You may have to pay more for quality & experience.
2. A lot of mini-labs make lousy negatives. While it less common to mess up the developing machinery, it's not unheard of. But if you shop around, again you might find a lab who cares. (Scratches and dust are more common problems.)
3. Light leaks as mentioned. This may not affect all exposures equally. The ones that spent the least time in the leak area (e.g. end of the roll where you rewound shortly afterwards) might still be OK.
4. Take more control by developing, printing (or - gasp - scanning) your negatives yourself. If you can't find a lab you can trust, then learn to do it yourself. Details of scanning are off-topic (see hybridphoto.com) but every step you do yourself means you know exactly how well (or poorly) it will be done.