Do you know specifically what kind of maples they are? Some types of maple trees drop the samaras in spring or summer and others drop them in fall. With the types that drop their seeds in the fall, they usually drop them before the leaves being falling from the trees in autumn.

Every now and then you'll have a year when the maple trees don't produce viable seeds. The reasons why can vary---it can be because a late freeze froze the flowers so that seeds didn't form or it can be because there was a very heavy load of seeds the prior year, which can result in few to no seeds in the year immediately after the very heavy seed year.

Dawn