Since you're not in flower this is more likely a lockout than a deficiency. Sweet spot for phosphorus uptake is between PH 6-7. Promix is acidic so I suspect your rootzone ph is a little low. Do you water to run off? When you get your pen, water to get enough run off and test. If it's super low... 4-5 I'd water at a PH of 7 to bring it back into range.
Personally if I were to ever run promix again I'd do my first couple watering at 7 just to bump it up a bit.
This is so early that you can solve this long before it goes into flower.