It may be that the silicon cord has worn and needs replacing.  This will depend upon the materials being cut and the age of the machine.

If the cord is not worn it could be contaminated with oil or grease of some kind.  Try this:-

• Clean the silicon cord in a bowl of diluted detergent, rinse it well in clean water and dry.
• Put some spirit (acetone, white spirit, etc.) on a cloth, push into the groove and work it up and down the groove to clean it.
• When dry place the silicon cord over the groove at one end and press it down into the groove.  Work along the length of the cord pushing it down into the groove as you go. DO NOT STRETCH THE CORD LENGTHWAYS.

If you do need to change it, just check it is yellow and circular in cross section, then order it as SILYR from Keencut or your local reseller.