Hi Nancy,
You can remove the cover of the carriage but to see the cams you need to take the carriage off the machine. If you do this whilst the cam is stuck, you should be able to see which one it is. It may be that it is sticky with old oil or fibres or sometimes the springs get pushed out of position. Loosening a screw slightly may be all that is needed to let things return to normal before retightening it.
While the carriage is off the machine, thoroughly clean the underside with surgical spirit (not methylated spirit), then apply machine oil on a cloth to all the moving parts and along the edges of the channels through which the needles pass. Finally oil along the front and rear rails of the carriage. Then after cleaning the needlebed, oil along the front and rear rails and across the needle butts. Do not use WD40 but a fine machine oil as supplied with the machine.
I hope you manage to get your carriage working properly.
Regards,
Sue, February 2011