Only a warm shower pipes are hot but shower barely gets warm

It is possible that the little O-Rings in the shower faucet are bad and not mixing the water properly.
You need to turn the water off to the shower, disassemble the faucet unit to see exactly what type of O-rings or washers it uses. Take the old ones to the store with you(Home Depot or an really good hardware store)and they can help you match the correct ones. Good Luck.

How long have you had this problem? Is it a new shower valve? Do you have a dial for mixing and adjusting the hot and cold water? Just a few questions before you pull it apart. Also, If you are the original owner and before you go and pay for parts, call Delta and describe your faucet and the problem. They may send just what you need if it is under warranty.