Serger blades do not cut.

Question

elna serger pro dex704: the cutting blades do nothing. Not old enough to even replace, no barbs etc., and have put in the 2nd blade (never used) to no avail. Have adjusted the moving blade up and down, no change. Sewing on cotton knit (t-shirt). Fabric seems to rag and not cut. All cleaned. Suggestions? SJ