Well it's not overfeeding, so we can rule that out.

Have you been giving them fert.? If not, that is a major deficiency. If you haven't been giving them food, they NEED it or it will continue and eventually kill your entire plant.

If it's in veg. right now, which it looks like it is, it needs some high in Nitrogen (N-P-K). There are 3 numbers on a container of fert., so any fert you get, make sure the first number, the N, is the highest number. Avoid Miracle Grow food as it isn't formulated for resin producing plants.

Your plant could also be rootbound. It looks like iut's in a really small pot. Transplant if root bound, and before you put into a new pot, take a butter knife and slice through all the roots. That cuts the bound roots and gives room for new root tips to develop.

Give us more info. on your feeding times for that plant and maybe we can help you out a little better.