My lawnmower won't prime?

Last week my Mountfield SP530 with Briggs and Stratton Sprint engine 10D902, cut out five or six times as I was cutting the grass. There were some up and down revs before it died each time. It restarted straight away on all occasions. However this week it would no longer start. It seems that no fuel is entering the carb when pushing the primer, just a rushing air noise. The primer button seems OK, and the fuel is fresh and contains additive. Could it be a simple blockage or something more serious?