I am facing a weird issue with MOSS 2007 SPD workflow for past couple of days. I had created a site from the Fab 40 Training site template. The workflows were kicked off correctly until last few days when the workflows have suddenly stopped sending mail. I checked that all the workflows that came with the site template are still sending mails. But all new workflows are failing. I created a new custom list and attached a new SPD workflow which fails with the error message "Failed on Start (retrying)". I checked the logs too but could not find any issues.
I also noticed that the workflow is checked out and I am unable to check it back in.

SharePoint is also not allowing me to check-in the workflow. Any help would be appreciated.
UPDATE: While debugging, I was able to narrow down the issue to the custom content types in my site. I had created a content type in dev environment and used it in my site to create a calendar. I then created a site template out of it and exported the site to production. The content type definition did not get carry over. Now whenever a new custom list is crated and a custom workflow is associated with it, the workflow will not checkin and fail on start. Any help?