You may add more P during flowering, but outside of cutting and seedling stage, I would bet your miracle grow is plenty sufficient in P for veg. Provided the soil isn't over 2 months old. Has it ever been transplanted or anything?

This is the primary reason I mention PH. Nutes can be present but the roots require about 6.5ph in soil for them to be accessible to the plant. Many growers misdiagnose PH imbalance as nutrient deficiency. The fact is, it's there, just can't be used.

Proper PH is essential to diagnose ANY deficiencies. It's like troubleshooting anything else, if the first link in the chain is broken, nothing down the road matters until you fix the first link. In this endeavor, PH is the first link.