I am renting an apartment, and I have the same problem- the hot water in both kitchen sink and bathroom sink is hot, but the hot water in my bathtub is lukewarm . I have brought this problem to the manager's attention, and he has sent the building maintenance guy up to my apartment several times to correct the problem to no avail. What steps do I have to take to resolve this ongoing problem (5yrs)?

If it is newer type of faucet than it is preset for lower temperature - especially if it is rental apartment. If you snap a pic of it and post it here, we could identify the faucet and then tell you how to get rid off that hot water restrictor. Back to you. Milo