Momentum not conserved for one of my class' experiments

What about the X values? Were they close together or far apart? If the time of flight varies significantly between the two marbles, so would the y value when they land. Are the marbles still on a track when they collide? If so, could that be affecting the y velocity of one or both marbles?

The initial and final values for the momentum in the x-direction were reasonably consistent.

As stated, the time of flight did not vary appreciably to the naked eye or ear.

There is a little screw with a dip in it where the target sits. The track does not touch the screw in any way, however the screw is sufficiently close to the track so that the marbles can collide. Although if you have any ideas about how this could affect the y velocities, I'd be interested in hearing it.