Try lowering the tire pressures aswell, this can sometimes offer more grip but only for a few laps. If in an open qualifying session (i.e. no superpole) try and make it so the tires are at their optimum temperature (around 90 degrees for most cars) on your third lap or second hotlap. Dont push the limit on the first hotlap and lock up/make mistakes as this ruins your momentum/ concentration and damages the tires, just get a feel for the car and hopefully on your second timed lap you'll be 'in the zone' and fully aware of the car balance/ grip levels which will let you hit your best lap time. With superpole it's a lot harder, you need to test what pressures get the tires to 90 degrees for your only timed lap beforehand and also learn what kind of outlap you need to do to achieve this, i.e. slow and smooth or hard and erratic.
Hope this helps.