hey man im no expert at all but things to look into about that would be ph levels, and nutes. what are you feeding them if anything? and whats your ph level at in your soil? post some pics for others to help you further. good luck.
well if you are now wondering where to pick one up, you can purchase one at lowes for like 8 bucks. it has a moisture meter and a light meter too. i have one from there and it works pretty good so far.