Upvc door water channel not draining

My parents are having problems with water leaking in through their back door. I’ve checked the base of the frame, and there are two drainage holes for what I would call the “outer gulley” which leads to concealed drainage to the step outside. Recently, I clean those drainage holes and - as much as I could - checked the concealed drainage points were clear. Then I dried the frame and waited a short while during very heavy rain, the took the photos shown. What puzzles me is that there seems to be what i would call and “inner gulley” and it’s that which was full to overflowing; and that has no drain holes of its own. There are two screws in it but I have not touched them. I am puzzled why there are two gulleys, and assume maybe it’s due to size of the door. The strip between the two gulleys seems to be a bit of plastic from the outer gulley which is raised and overlaps slightly the inner gulley. Perhaps the frame is not level so the water from the inner gulley reaches the inner edge of the frame before it can spill into the outer gulley and drain away .. but I don’t see the reason for the division between the two gulleys. I’m assuming it would be unwise to do any drilling, but I have wondered if I took a Stanley knife to the middle raised plastic - carefully - whether I could create a gap for the water from the inner gulley to flow to the outer one? Also, I think the inner gulley is probably filling up first anyway but I found it hard to be sure where water was entering. Once the weather improves I may run a hose / sprinkler against the door to try and find out. Anyway, if anybody is able to explain the two gulleys and whether there should be drain holes for the inner one or whether it should drain into the outer gulley .. or any other advice .. i would appreciate it!