*istDS + EX150A flash

Is anyone using their *istDS with Interfit EX150A flash units using a hotshoe/pc adaptor and sync lead and if so, do you have the same compatibility problem as me. Once plugged in and powered up, the flash fires once only and refuses to fire again. If I remove the sync cable pc plug and then replace it the flash then again fires once only. I've contacted Pentax about the problem, but they don't seem interested because I'm not using Pentax flash which I think is a bit narrow-minded of them. I'm awaiting a reply from Interfit..