You need two things for a leak free roofing:
1. The underlayment and flashing needs to be water tight.
2. The tile and water channeling needs to keep water off underlayment.
Here we have clogged metal channel by skylight. A cracked tile diverted water to side entering a few holes in the under layment and rain would drip. I put a new sheet of 40 weight tar paper by side of skylight under channel to fix leaking. I used peel and stick type asphalt shingle but normal felt or tar paper will work. You need a thick layer of Henry’s roofer patch under and all around edges. Do it yourself and save!