Mophead (I am assuming that is what you have) blooms perform best in dappled sun or in morning sun - afternoon shade so, try proving shade by 11am or earlier. More shade can be temporarily provided with an umbrella or chair. Very common summer problem in newly planted shrubs

It may also need more water so try providing 1 to 1.5 gallons of water in the summer.

It is not used to being in your garden and prefers the really shady spot in the plant nursery so now that it is your garden, it suffers from what's referred to as transplant shock. Once it becomes established, these problems should be reduced but confirm that it gets enough water and sufficient shade. Mulch is important too as it prevents loosing the water thru evaporation; maintain 2-4" of mulch all year long.

If I dead head (cut off at the next leaf down?) will it bloom again this year?

I would expect more bloomage from rebloomers only.

But be careful. If you interrupt the Spring 2017 flower bud development with pruning now and it is not a rebloomer, it will not bloom again and you may loose the Spring 2017 flower buds. The 2017 buds develop sometime in July thru August. Maybe try it with only a few stems to see what happens?

Any blooms on anything will eventually turn brown. August is 2/3 thru summer and this is a regular occurrence.