Flow limitations are like partial breaths, if you are having lots of them then they can disturb your sleep. Increasing the pressure sometimes helps but not always. What does help is a larger pressure difference between inhalation and exhalation pressures. On your current machine the only way to do this is with EPR which is correctly set to 3 which means the exhalation pressure is 3 cm less then inhalation pressure. One thing to note is that minimum pressure cannot go below 4 cm which is why a pressure of 7 will help since it allows for the full 3 cm drop whereas when you run at a pressure of 6 cm you are only getting 2 cm of relief.
The only thing you can try on your current machine is higher pressure which you don't seem to like, the only other option is to get a machine that is capable of a larger pressure difference. That would be a bilevel machine such as Resmed Aircurve Vauto. The only way to get a bilevel machine is by prescription through your doctor or by buying used. To go through your doctor you would want to ask him to review your data specifically looking at flow limitations. You will need a decent doctor in order to convince them to go to bilevel though as not all believe in treating flow limitations with bilevel. The other thing that may support that your breathing is restricted is that your tidal volume and minute ventilation are on the low side, this is either because of restriction or because you are of smaller size (like around or under 120 lbs).
If you post a few 2 min duration screen shots showing breathing when the machine is not scoring flow limitation then that will help understand if breathing is being restricted but just not being scored as such. Sometimes breaths look flattened and the machine doesn't score those as flow limitations.