When I first got my BNs I was worried about them too. They had eaten all the algae and seemed to have no intereste in the zucchini and cucumbers I put in the tank for them. I was afraid they would starve.
I kept putting the veggies in every day, then after about a week I saw both plecos on their veggies.
I always put the veggies in the same place, and now both go running to them the second I put them in.
I think it's probable they've never had these veggies so it takes a bit for them to know it's food. I also like to break up an algae wafers and stick the pieces in the slices of zuchinni and cuke so they get more nutrients.