Skipped stitches and getting worse

Question

My stitches started skipping about a year ago. I took it into the dealer I purchased it from and they serviced it and said it was fine. It has continued to skip and now skips all the time. Not only on stitch 12 like before but also my straight sticth, my zig zag, etc. I am extrememl;y frusterated by this and am in the process of sewing over 50 costumes right now and its not working correctly.