Heat stress in the form of just plan heat, lack of moisture, too much strong summer sunlight, etc tends to cause issues for Limelight. It sometimes ends showing its feet because the bottom leaves fall down. Insert a finger into the soil to a depth of 4" or so and water if the soil feels dry. If the leaves are wilting, monitor the soil moisture closely (usually only a problem in the summer or when very windy).

Limelight may have been treated specially well somehow by the plant nursery to produce those big blooms you saw but, now that it is on the ground, it may be busy developing roots instead. So, I would give it time to become established. It is quite a tall and vigorous fellow normally; but do not try to fertilize it too much or the excess of nitrogen in the soil may result in less blooms.

I wouldn't do any pruning for next year since it is a relatively new plant. I try to give a plant 3 years untouched if possible since the leaves create food and help the plant grow, including a strong root system. (Though established H. paniculatas regrow so fast that it may not be an issue here.)

I agree that they look water stressed, perhaps due to not enough water, or perhaps simply due to an immature root system. I'd stick my finger down deep into the soil close to the base of the plant to check moisture levels and water long and deep if it doesn't feel damp. Once they are established I don't need to water my panicled hydrangeas, but yours is still new. Also consider your geographic location (Luis in TX finds his need more water and more shade than mine in NH. I don't water after year 3, and they are in full sun, though well mulched.) Where are you? You may want to give yours some temporary shade in the form of a tall chair placed to the south of the plant, or a sheet on tall bamboo stakes. Once the plant is fully established you shouldn't see this evidence of water stress for the plant, and if you do, move it to more shade.

Thanks! Im in zone 6 in Mass but these do get some strong sun from 7AM to around 4PM. I water them often but probably not as long as the need. I really want the blooms to be larger so I was thinking I would need to prune out some of the smaller branches next year.

Before stopping watering or watering more, check how the soil feels. I know that here in NH we have had quite a lot of rain, at least an inch each week except for two, and some weeks as much as 2 1/2", so if your rain has been similar, it could be too much moisture. Often too much and too little water look similar, but you won't know unless you check the soil moisture yourself. Generally I agree with springwood - paniculatas need no supplemental moisture once established, but yours has less than two growing seasons in the garden, and it is a large shrub, so it could be either too much or too little water at this point.

Gardening is a lesson in patience. Let the plant settle in for at least three seasons before trying to force it to have larger blooms. After next season prune to increase bloom size.

Limelight leaves will flag (go down) at a 60 degree angle in the sun and heat, but should mostly reset by the next day (except at the very top).

If the leaves are flagging, touch them. If they still have "beef" (feel like rubber or plastic - not limp/bendable, or crispy) the plant does not need water.

The best way to tell dehydration is if the blooms themselves are flagging (flowers extremely limp and crinkly), the current year growth stems are bending / going limp, and the leaves are also going limp.

Once again, the leaves remaining rigid and simply bending downward is normal. Keep an eye on the differences between dehydration (problem) and regular flagging (no problem).

[Overwatering not only causes bottom leaves to go yellow and drop at an accelerated rate, but as shown in the ^ pictures there also looks to be a fungal issue now present where the leaves are turning brown from the edges inward. It will also spread from the bottom of the plant up. Manually get rid of all affected leaves and leave it alone for a while. Worry not, fret not]