I'm at the 5 day mark on a 1.072 DIPA using WLP090 and down to 1.028. I stepped up a small vial that I saved from a previous starter I made. Fermented between 62-65F. There's still krausen on top. I shook to aerate and was under the impression I used the appropriate amount of cells of 090. I'm scratching my head. Wait it out? Pitch more yeast?
My experience with that yeast (dozens of batches) is that at 65-68 (recommended range) it ferments fast and clean. At 64 the pace drops off considerably, and at 63 it just about goes to sleep. Try heating it up and swirling the carboy/bucket a bit. I've had krausen linger at the top of the beer for weeks on end with that strain, though not every time, so I wouldn't read into that so much. But at this point, don't be afraid to ratchet up the temp to 69-70 and swirl the carboy/bucket. You didn't say what temp you mashed at or if there's any simple sugars/how much crystal...but all those things play into it as well. This yeast strain is a beast in it's range, but it's S.L.O.W. if you go below 65 degrees.
Edit: reading back I can't help but notice how many people say they're fermenting this yeast below 65. This is just one brewers experience (this post) but I can confidently say I've used it in at least 30 batches, so my experience is considerable even if it doesn't seem to match that of others for some reason.