sometimes bathing can make mats tighter- so try that as a last resort- as it will likely not make the bunny very happy and you still have the mat to deal with. I always advise caution when using scissors on any mats- it is best to use either a mat rake
http://www.drsfostersmith.com/product/prod_display.cfm?c=3307+5+56+843&pcatid=843
Electric clippers are also a good tool – as you can get between a mat tight to the skin without cutting the skin. (like clippers dog groomers use)
IF you ever use scissors on any of your pets- I would strongly advise doing it while your vet it open!! An accidental cut after hours can mean an emergency fee plus others fees (sedation, wound treatment, antibiotics, pain meds). You would not believe how often people accidentaly cut their pets skin with scissors while grooming- even though they think they are being ultra careful- it still happens. I have seen these type of visits cost easily 4 times what one professional grooming would have cost.
There are alot of different textures to fur- some short, some long, super fine, silky, soft, wavy, coarse- and they each responds to grooming and mat removal a little differently.
What type of bunny do you have? What type of fur? where is the mat? Is it related to shedding, the animal not grooming, or a messy bum?
Julie