What kind of soil did you use? Be really specific. Having a good soil mixture to begin with will help insure a healthy plant. If you start with a bad soil mixture, no amount of tweaking nutrients will held. You really also need to know something about your water quality. Ph that is out of range can affect nutrient uptake. Too much of one thing can lock out others. Ph can be very important.
How large are they? You might actually want to repot with a good quality soil mixture and then use the right nutes. There is really nothing about a 20-20-20 fertilizer that is good for mj. First, 20-20-20 is quite strong, but more importantly it does not supply the nutrients in the correct concentrations. During the vegetative stage you want something with more N and K and less P. For instance the GH veg nute that I use is 7-4-10. Flowering nutes will have more P, and less N and K.