Dolomite lime is used to raise the ph of soil. Soil is normally made from peat, which is very acidic (ph less than 5.0), so they add lime to bring the ph back into the 6's. Either your soil didn't have enough lime to start with, or it is now running out.

Cold night time temperatures can also cause the newest growth to be yellow. Is it possible that the temps are getting below 19?

Some pictures would help if possible. There is a copy-and-paste troubleshooting form at the top of this section. Filling it out here makes it much easier to figure out the problem. Can't really give any advice without all the necessary info, like watering schedules and what nutrients you are using, and how often, etc.