Other than under-watering or over-watering, which will kill any plant, weigelias are susceptible to a crown gall (look for a gall at soil level) and a root rot.  Since the shrub probably has not put a lot of roots into the surrounding soil yet, you could possibly dig is up gently and inspect the roots.  Healthy roots are white or light colored. 

This shrub needs sun and any surrounding plants should be removed. 
ECN