Why are some of my tomatoes not ripening? They're growing in pots on south facing patio. Compost is from my horse manure/compost pile. Lacking potassium?

Tomato fruits ripen based on a number of factors. One would be the characteristics associated with the variety - is it an early or full season variety. Another would be the amount of sunlight the plants receive each day. Full sun will ripen before plants grown in semi shade. A third would be the daily temperatures we have had. Cooler temperatures slow the development. If the plants have received too much nitrogen, flower development is also delayed. If you have had good flower production, I doubt nutrition is the problem.