Aquatic plants, good or bad?

So a river I trout fish, has had some aquatic vegetation growth over the past several years. The water flows are great and there are an abundance of smaller trout. However, it seems the larger trout are harder than ever to find. I have nymph and streamer fished it with much less success than years past. I was wondering, if the larger amount of plant growth can have a negative impact on the fishery. As stated, the flows are great and there are white water areas with plenty of ripples as well so I am not sure if its an oxygen thing. What are your opinions and experience? Is this a concern, or do I just need to step my game up?