Dryness at the roots is the usual cause of this - assuming the pots are free draining with holes in the bottom, are you watering till the water runs out? Move the stones aside and check how damp the soil is beneath, but these signs may indicate a period of drought a short while ago rather than now. This assumes there's no difference in the compost in each pot, i.e., they were both planted into clean containers with fresh compost.

checked soil beneath stones - warm and damp. have since given both plants a liquid feed and will see what develops over the next week.

at last some one who understands, i have done exactly the same thing as you with the same results i have fed them and watered as you should but still going brown,wondering if i need to put them in the ground rather than a pot.