It's almost always a matter of pH lockout. No idea what Lowe's potting soil is....but my guess is probably not great....and likely to be acidic (since most people who would buy potting soil from Lowe's are probably using it to start their tomatoes). I'd like to see the label on that bag to check what's in it.
Yes, you need to test the pH. You can take an empty pot filled with the same potting mix, and run water through it. I prefer to use the General Hydroponics test drops because most of the pH meters aren't accurate. The test drops are cheap and easy and much more consistently accurate.
You should also take a couple spoons of the potting mix and put it into a clean jar with some distilled water....swish it around and allow it to settle. Once the "slurry" settles, test the clarified water with the GH test drops. This is called a slurry test.
My guess is that your potting mix is too acidic. It's a very common thing with bagged potting mixes. IF this is the case, then you can add some "pH UP" to your water to bring the pH to slightly higher...say, maybe 7.5-8.0....and then test the runoff/slurry test, again, until the runoff reads somewhere around 6.0. After that, make sure that every subsequent watering/fertigation gets pH-adjusted to somewhere around 5.8-6.5. You should also regularly check the runoff throughout the grow.
Always check the weight of the pots when they are dry as well as when they are wet so that you don't over or under water the plants. Never go by a set schedule because a sick/recovering plant isn't going to be drinking as much as a fully-healthy plant. If the pots are light, then they need water. If they are heavy, then they don't need water.
Also, don't overdo the nutrients.
