G'day Gwen. Welcome to Apnea Board.
All masks leak to a certain extent - in fact they are vented to allow your exhaled CO2 to escape. This is called intentional leak. In addition, you can have unintentional leak, which is just what it says. Resmed machines allow an unintentional leak of up to 24 L/minute. Above that limit the air pump can't keep up and the machine can't properly diagnose apneas and hypopneas.
The S9 will give you a red frowny face if you're above the red line for 30% of the night. That means for at least 30% of the time the pump can't supply enough air and the machine can't properly diagnose your events. So a low AHI under these circumstances can be quite misleading. In my view I think the 30% is setting a very low bar, and even at 24% of the time you're losing too much therapy time. (And apparently SleepyHead agrees). SleepyHead defaults to the same 24 L/min leak rate that Resmed uses, but I don't know at what point it gives you the excessive leak warning.
So you really do need to manage the leaks to get a proper level of therapy.
If you're using a pillows mask it's possible the leak is coming through your mouth, in which case you might like to try a chin strap to gently hold your jaw and prevent your mouth falling open. Some members use the "tongue stick" method which will seal the mouth even if it falls open. There is also a rule of thumb which says you should use pillows one size larger than what seems right - but in your case you got better results by going down a size, so that doesn't always work.
The other thing you could try is a full face mask, which takes care of mouth leaks and might also help with the winter congestion.
If you post a screen shot from SleepyHead, that will help us see exactly what's going on and give you some better targeted advice.