Why is there no hot water coming out of one shower and the kitchen sink when all other showers and sinks are getting hot water?

Try removing and cleaning out the aerator on the sink faucet, and the shower head.

(Word to the wise: if you take them apart, write down the order. It can be tricky trying to figure it out, afterwards).

By just removing them, you should get hot water, thereby verifying that these are the cause of the problem.

If you don't want to clean them, you can always replace them. Water saving aerators are available for faucets. Water saving shower heads are also available. Some shower heads even have on and off switches, for additional water savings.

How I learned, the hard way:

My situation was, hot water from kitchen & bathroom sink faucets, and even from the tub faucet, but not from the shower head.

So, I removed the shower head and soaked overnight in CLR.*

Cautionary note:

Probably would have been OK to soak overnight in vinegar, or for a few hours in CLR.

But overnight was too long in CLR. It must have dissolved more than just the calcium deposits. Now the shower head has much thicker and much more water consuming streams. (Previously it had a very stingy water conserving spray).