My first thought on looking at these pictures is, 'Are they really lilacs?' Do you know what cultivar they are meant to be?

Then I'd dig down carefully and look at the root ball. Is it wet or dry?

looks like your weep hose is failing at providing proper water ... see link in regard that topic ...

take a hand trowel.. AND FIND OUT HOW DRY THE SOIL IS.. AT 2 TO 4 INCHES ... in the root mass planted.. and in the soil outside that area ...

be nice to know where you are.. and what the ambient weather has been since end of winter ...

i also dont think they look like lilac ... maybe viburnum??? .. but it really doesnt matter as to the advice given .. a shrub is a shrub ... excepting the foo foo ones.. and this isnt one of those ...

ken

btw... SE MI has been in severe drought .. my lilac were on the edge of not looking much better than this one ... this doesnt worry me much.. if you can perfect watering from here forward ...

https://sites.google.com/site/tnarboretum/Home/planting-a-tree-or-shrub

They are lilacs at least according to the tag . We are zone 5 north Colorado. It's been hot 90s and relatively dry which is typical here. I will try digging a little to see what the roots look like

According to the tag they may be Lilacs but according to the leaves, bark and growth habit I am still dubious. If you Google some images of lilac leaves you'll see what I mean. Tags do get mixed up at point of sale.

i think it looks like a moisture issue so checking the root ball is a good first step.

That is definitely not a lilac! floral_uk has a point, I bought some knockout roses and one had 2 tags on it.