My well water is quite alkaline, so it's vinegar for me. My light's a tanning light, quite cheap from someone who wanted rid of it. I found paper can make quite a difference. Stonehenge is my favorite so far for cyanotype, though I don't like it for other alt processes. Another possibility is that you're just not exposing long enough.
No one's mentioned the glass. You don't want the expensive kind that blocks UV light.