truss tube collimation problem

I have just complete & collimate a truss tube scope (3 truss pole).
But when I tilt it towards horizontal position, the primary mirror
collimation was somehow 'shifted' in one direction. It seems like
somebody had adjust the primary mirror collimation screw. I've checked
that all screw are tighten & the primary mirror was not shifted, but
still unable to track down what went wrong. As the side bearing is
quite low, I employed a spring mechanism to counter balance the scope.
When I remove the spring mechanism & hold the upper cage as I lower
the scope, the 'shift' in collimation seems to be slightly minimize.

Can someone enlighten me? Thanks.