Have the trees grown up to make too much shade around them?

Some roses, typically older varieties, are single bloomers, producing a flush of blooms just once a season. Without knowing exactly what type of roses you have, that's a possibility.

Otherwise, it is virtually impossible to damage a rose by pruning :-) you can chop them down almost to the ground and they will respond well. Of course pruning at the wrong time could also limit flowering or remove buds. Late winter/early spring is the recommended time in most parts of north America. Roses tend to be heavy nutrient users and appreciate fertilizing on a routine basis. And they like plenty of water. Sun, ferts, water and decent soil and they should be good to go.