RC paper negs for UV printing

Hello,

I'm quite new to alt processes, still learning and experimenting, though I am now making cyanotypes from regular 4x5 film negatives quite regularly. Recently, I've read in several articles claims, that it is possible to make prints using UV light source and PAPER negatives. That was quite interesting to me, because I find the 4x5 images just a wee bit too tiny. Having built a 5x7 pinhole, I tried to make a cyanotype print using the paper negative on the standart fomaspeed RC paper. And it just did not work - my usual exposures with facial solarium are between 3-7 minutes according to film and distance, the paper neg was exposed for more than 25 minutes and VERY close to the UV source but the image was barely visible.
How comes this? Are only ink-jet or laser printed paper negatives to be used for UV printing? Is the standard RC paper too thick or does it have something that would block UV light - since contact printing paper neg onto another RC or bromide paper is not the issue?

All suggestions would be very welcomed.
Thank you.