The reason I asked about smelling the reducer is I had this same thing happen to me on a PPG job. I was using their "supercharger" hardener DFX7 but what clear I used has escaped my memory( I tried like hell to forget this nightmare!!). Anyhow, I cleared the job, baked it for 30@140(total booth time 40 minutes) and when I tried to unmask it the damn thing was just as tacky as a ten minute air dry! So I baked it again, this time 60@140(boss was happy about the amount of gas I used to do this) opened the booth and still not dry.Let it sit for a day with no change. Called the PPG instructor at the training center he came up and scoped it out. He told me to wet sand it to open it up and when I did the smell of reducer was strong. Long story short, car had to go so I recleared it and come to find out I was not using an accurate mixing cup and was putting in more hardener than needed. The instructor said that the clear skinned so fast and that the skin got so hard it couldn't leave the solvent pop marks associated with trapped solvent because of the extra hardener. Remember,the skin got hard and thats it. Hard enough to sand but yet wet enough to print. Hence,trapped solvent and could smell it when opened with wet sanding.
Anyhow,this is just my experience, wheter he was right or wrong thats what we accepted. Good luck!!