Walk-Behind Mowers

In April, I bought a walk behind Poulan mower from the Shaw AFB Base Exchange. I was then sent to Minot AFB for just over 2 months and while I was gone my son told me the mower would start but stalled as soon as it was pushed. I returned 1 July and brought it back. They did not want to exchange it but did because I had noone who could bring it back sooner. On 2 July they made an exact exchange and told me I could not exchange it again because they were doing me a 'favor'... Two weeks ago, this mower started doing the same thing - it will start, but stall as soon as it is pushed. I worked well for about 5 mowings. I use the push mower only to go under trees and close to the house so it does not have much to do. Help? Thanks. Nancy Ray