Interesting network problem

I have a wireless router, it has a local IP of Two computers are wirelessly connected to it and while they get their IP via DHCP, the router has them in his reserved DHCP clients list so the UPSTAIRS computer always gets and DOWNSTAIRS computer gets
Anyway, they can both ping the router without a problem. DOWNSTAIRS can ping UPSTAIRS but UPSTAIRS can’t ping DOWNSTAIRS. What the hell is up with that huh?
It’s interesting, I know how to plan big static networks and stuff, but I can’t solve these stupid home network problems. Must be a sucky router or something. Or something wrong with the downstairs comp.