Our home is 3 years old. We have a bathroom upstairs that is hardly used. For some reason, if we do not turn on the hot water in the bathtub every month, there will be almost no water pressure when I turn the faucet all the way to hot. This has actually happened several times and someone came out and fixed it, but I still do not understand why this occurs. It is an upstairs bathtub... the faucet in the sink is fine, but the tub is not. Of course the sink is used more ofter. Can someone explain why the hot water pressure goes away after non-use? And, how do I fix this, once it occurs?

What type and name of faucet?

I'm not certain about the type. The brand is MOEN. You turn the water on by using a single lever from the bottom position (6 o'clock) and turning it counter clockwise. The further you turn, the more hot water is dispensed. I tried to find a picture of it, but I'm having a hard time finding one.

It is a moen positemp. The plastic handle takes a phillips screwdriver to remove. The metal handle takes an allen wrench to remove then a phillips screwdriver to remove the metal end piece connected to the stem. Then remove the inner cog which is a white plastic with gears and rotate it slightly to the left. This will allow the handle to rotate a little bit more to access more hot water. Reassemble handle after verifying high limit will let you turn it far enough to hot that you can keep your hand under it but is very uncomfortable. Anything hotter is a scald danger.
If this does not remedy your situation then you need a new cartridge. I can advise on that later.