Yellowing leaves is a common symptom, which can be caused by different things. Some pictures would help.
In the meantime my first thought is the grow bags might be holding too much water. Have you checked to see how wet it is inside them? Overwatering can produce yellowing leaves. Here's a little bit about overwatering:
The symptoms of overwatering usually start with the plant looking a bit wilted, particularly during the warmest part of the day, although they usually regain some turgor at night. This can induce a cycle of more overwatering as the grower assumes a wilted plant is not getting sufficient moisture. Pale green or yellowing on the youngest leaves also suggest overwatering, as iron is the first element that a water-damaged root system fails to take up in sufficient quantities.
Along with cutting back the watering if this seems to be the issue, you can add a bit of ironite, which helps the plants green up again.
If leaves on the whole plant are pale yellow, this can be a symptom of nitrogen deficiency. If you've mulched with bark or leaves, especially if they've been turned in to the soil, it can use up a lot of nitrogen trying to break it down. In which case fish emulsion or other nitrogen sources should help.
Here's a source about problems in tomatoes, but it gives a list of different symptoms and what might cause them that seems helpful https://yardener.com/YardenersPlantHelper/VegetableGardening/VegetableFiles/Tomatoes/ProblemsofTomatoes