Are you turning on the autopilot after takeoff?   If you ask the autothrottle for takeoff power, you rotate at VR, then you have to manually adjust pitch to maintain v2+20.  That should be what the flight director cue directs you to do.  Once you turn on the autopilot, you can use VNAV or FLCH to have the autopilot take over the climb.  The autothrottle will switch to climb power automatically.

*Edit....
In order to have the flight director cue direct you to maintain v2+20, you have to dial that speed up in the speed window on the MCP.