Arugula not growing :(

I planted arugula and salad bowl lettuce in my SFG this February. (Both packets of seeds I used said they were heat tolerant varieties - suitable for zone 8 and above. I am in zone 9.)

The lettuce is doing ok - (not great or anything - but it's growing.) It'll be 5 weeks today, so they're supposed to be harvestable in 2 weeks. The arugula, on the other hand, is miserable. Shoots are barely out of the soil..and I only have maybe 2 plants that have gotten to the true leaf stage.

What happened? I fertilize every other week and keep the soil moist.

Unfortunately, when we planted our SFG back in Feb., it torrential down poured the following 3 days. (I mean terrible storms - crazy wind and everything.) I know we probably lost some seeds because of that, but would that have had any impact on the plants that stayed behind? Everything else seems good except the arugula..

Any suggestions?

Arugala is the same thing as the weed called rocket. I generally pick it wild unless I buy mesculun seed mix which invariably has some in it.
It wants good drainage, loamy slightly acid (6-6.5) soil and a bit of shade, though a good many sources tell you to plant it in full sun I very much doubt that that would be a good idea in your zone. Perhaps add a bit more organics to them and give them a parasol equivalent?

I almost always plant my salad greens in partial shade. They are a wee bit slower growing, but that is actually good thing as they don't bolt so soon.