There are many reasons possible. If the plasmid puriication is poor and too many salts remain then some enzymes can be poor cutters at the wrong pH or salt conditions. Also there may be enzyme inhibitors from the bacterial dna left in the plasmid dna after chemical purification. The plasmid may be coiled so as to hide the cut site in the middle of the molecule so try cutting for longer and a slightly different temperature. Check the quality of the enzyme by cutting another sample of any dna/pcr product/other plasmid to make sure that the enzyme can work. Try cutting with more enzyme, Run a sample of your uncut dna on a gel to check the purity of the plasmid dna and try cutting with another enzyme just to check that the dna is of good quality.

I agree with Paul Rutland that dirty DNA is often one of the problems. I would also be sure that your enzyme is not affected by methylation (dam or dcm), there are a handful of enzymes that are so impacted. Lastly be sure that you actually have the correct plasmid that you think you have!

Carefully confirm that your plasmid is not contaminated and,/or not misidentified.

Thanks