Can not get handshake

[outcast]

hi. im using backtrack 4 final release. and i want to hack my modem (for learning wireless hack). i set my password wpa-psk and connect 1 pc to modem and create more traffic. but i can not get handshake from modem. i done everything what aircrack web site says. i have intel 3945 abg wireless card. i already crack wep key with it. what can be the problem ?
the acks always showing like this
[2/54 acks]
[10/54 acks] no zero acks on modem.

[pieface]

You need to deauth the connected PC,if you cannot deauth it,just reset the connected PC's wireless connection to simulate a deauth.

[outcast]

You need to deauth the connected PC,if you cannot deauth it,just reset the connected PC's wireless connection to simulate a deauth.

thanks but this is not an answer for me. why i can not get handshake using deauth attack ? what can be the problem