I bought emulsion for cold light exposure, but I can no longer get the exposure timeframes exactly right. If I expose the frame for 15/20 minutes the lines printed on the gel aren’t visible. Perhaps the stronger the light, the shorter the exposure time, is that right?

That's correct, the stronger the light, the shorter the exposure time.
Therefore, if the lines aren't visible, the exposure time needs to be reduced.

I don't know anything about the exposure system you're using, but we know for sure that, for the same exposure system, the ready-to-use emulsion for white light needs to be exposed for approximately half the time with respect to a diazo emulsion.

I suggest trying an exposure time of 10 minutes, if the lines still aren't visible, try 5 minutes.

If the exposure is insufficient, you can always increase it until you find the correct intermediate time.