Living Soil is cool, but plants will suck all the nutrients out of a small pot in 3-4 weeks, unless you top dress or feed in some way.
If you're 3 weeks into flower your soil is depleted, and your probably right to be feeding some nutrients.
Keep an eye on that little struggling plant for a day or two. Ideally, it was just hungry and now that you've fed it it should get a slightly darker color and the leaves should perk up. And the plant should look "better." Then from there, you can keep doing what you're doing.
If it gets More pale and lime green, and the leaves get worse (even though you just fed it) you know there is some problem. Like lockout or out of range pH or something. Ans you'll need to flush out your soil or something.
Theoretically, it's also possible there is some problem with your watering system. Check the water in your reservoir. If it's stinky, slimey and funky it could be causing issues. If it's clean and clear it's probably nothing to worry about.