Shouldn't be a problem with the ProMix, at all. Perhaps the medium wasn't hydrated well prior to using it? If not, it could be resistant to wetting. Or you aren't using enough water per application?

Thanks rhizo_1! I didn't see how there could be a problem, but I'm getting ready to start a hoard of seeds in the upcoming weeks, & I wanted to be sure I was using a good medium...

I like to have pro mix on hand in spring so I have over wintered my promix BX for years and never had a problem. It sits in the garden shed all winter. As long as it is kept dry it can last for many years.

Thanks to two folks that know their stuff! So nice to have both of your inputs (& I gotta mention Dave too) on Garden Web! Although I've been starting seeds for 5+ years, I still find myself second guessing my methods......

You're probably seeing the "dry sponge" effect. If you have a sponge that is very dry, when you first put it under a stream of water, the water will run off. You have to soak it a bit for it to regain its water holding properties. Peat is the same way.