make sure heat is turned off.. media will not always rest flat so some deviation will be expected. Skipping is usually attributed to too much or too little force. If it just appears to flat out not cut certain lengths than the speed may be too fast and not enough pressure applied.
Additionally, the cutting strip may be damaged in those areas causing the blade to skip and ricochet around.
Lastly, the blade tip may have broken off- if so, the blade will always give an unreliable cut.
I have a bad spot on my cutting strip that I can avoid by centering the media, but if you know how your cutting strip acts, you can power through it- just depends on how much you want to put up with it.