I disagree on infection causing the low FG.
Even the *accidental* strains of brett take quite awhile to achieve super-attenuation (fermentation of normally unfermentable sugars). How long does it take to get to your FG?
155F isn't THAT high of a mash temp. If you aren't using any crystal malts, I wouldn't be surprised to see a FG in the 1.008-1.009 range. This is especially true if your mash temp falls a few degrees during the mash, it takes awhile for you to lauter, or if your mash thermometer is off.
I would check out the thermometer first. Then, think about mashing a bit higher. To get in the 1.015 range for my English Pale, I mash at 158-162 (depending on target starting gravity, amount of crystal, etc.).