I guess it could be any of a few things:

* Interference from a nearby access point on an overlapping channel.
* Your own access point is positioned poorly.
* You are covering the N900 wifi antenna with your hand.
* Your access point is under load, not able to properly respond to pings. (P2P can do this)
* Your AP radiated power setting is too low
* You are simply too far away from your AP to establish a good quality link.

Given that some pings are getting through, it's exceedingly unlikely to be a hardware fault. More an environmental problem.