It sounds like it is freezing up. Set the output temperature a little bit higher for a few trips and see what happens. I know that the Old Air Products unit has an adjustment to prevent that, but I am not sure on tye vintage air.
If the charge is too low, that will happen. I would recommend getting a set of gauges and setting the charge properly. The sight glass method is slightly better than squinting over your thumb at it. Yes, I know Vintage Air recommends it, but it is still wrong.
