lukewarm water to second floor shower/sink

we have a direct vent gas water heater and it works great. On the mainfloor the hotwater is ideal. Our problem is that recently the hotwater on the second floor (shower and sink) the water is lukewarm. I even try the shower with no cold water and all that comes out is lukewarm.

In terms of the piping, the hotwater comes out of the tank and goes to a splitter that brings the hotwater to the main floor (2 sinks, and a dishwasher and all working great). I think it is a "T" splitter with the other pipe going upstairs to supply the hotwater for the upstairs bathroom.

So I am not sure what the problem is because the hotwater is working...but for whatever reason it is not making its way to the second floor (which is wierd because it used to)

Any suggestions????