i had this problem with my ion a while back, best way i found to fix it was to replace the battery, and if that didnt work, to reset the dwell, or raise it until the marker cycles properly.
Hope that helps
Matt
"If your ion is doing weird crap, then its probably just the battery, dwell settings just dont change on their own"
The only other reason ive had for a similar symptom is when you go from a LP reg to a HP reg, and put too much pressure into the gun, and it wont fire unless u dial it down.