Mold inside windows, badly sealed windows?

SO...

had our windows replaced a few years ago and the min bedroom ones always had a bit of mold inside on the sill, they've been out and looked at them various times and although improvements were seen there's on that really still quite bad.

they did seem keen to sort the issue, been out a few times etc. but I wanted to get an impartial opinion rather than trust anyone that might have reason to make excuses, so thought i'd ask on here.

Are there any drain holes under the window frames (UPVC) that might be blocked and are causing water to collect? reason for asking is the window has no external sill as it sits on a stone sill instead. underneath they look to have been sealed between stone/frame.

I thought maybe on some windows that they'd drain into the sill from inside the window frame, i'd seen pics online showing this, maybe this is the issue (no plastic sill and sealant holding back water??)

EDIT - so i had a look and there is a sill , just not a huge one. there's also some drains been added to the frames since (during one of the repair attempts)

Can’t comment on whether it’s anything to do with the window cills but in my experience mould is normally caused by a lack of room ventilation. If you have trickle vents keep them open all year. If you’re getting condensation on the glass then it’s almost certainly a ventilation issue. I have a rental property & one set of tenants had massive mould problems throughout the flat. I knew it was caused by excessive heat & lack of ventilation but they wouldn’t listen. They moved out & new tenants moved in & have had no mould problems whatsoever since.