I don't know what Petsmart has, but can you post of pic of yours? Bacopa caroliniana and B. monnieri are two of the most common ones. I had the hardest time with B. caroliniana until I got CO2. Sometimes stem plants float up and don't stay anchored when they don't have enough light, IME. You could try to place it where it gets the most light in the tank. The Bacopa will eventually grow roots. In the meantime, you can bury the last leaf into the substrate and see if that helps to anchor it. That's what I do and it doesn't hinder their growth. I trim mine almost every week in my 5 gallon. I cut a few inches off the bottom, including the roots, if it's too tall for the tank. Then I anchor the stem in the substrate using the last pair of leaves, and it continues to grow, until the next time it's trimmed!