Guystapo,
I own a Cal not a Catalina, so for what it is worth, some drains use a flapper in the line. It is designed to allow water to drain out but attempts to stop water from coming up the scupper tube in a following sea. Some are made with loops so that water has to go up and over the loop to get out. Another one of those designs to minimize water flowing from the outside into the boat through the scupper. Still there are the WTF designs where the scupper is connected to internal plumbing drains. Run water into the scupper and if the thru hull drain is closed it backs up into the kitchen sink or some other place to be discovered.
I would encourage you to explore all of the plumbing lines from the scupper to where it empties overboard. If you see any connections in the hose other than where the hose connects to the scupper drain or the thru hull overboard you may find a flapper/check valve or a place where debris is blocking your hose. Maybe a lost bikini part from a previous owner.
If he hose connects to another hose not the overboard thru hull then you may have a WTF design system like mine where the water drains through some place like a sink drain. Then you have the opportunity to design your own solution.
I share mine as I get around to fixing it. Till then I keep the drain through hull open when it rains.
You share yours?
