It is often more easier to judge if the top is open or not from a chase boat.

If it is not open:
Was the rotation in/out? On a wing mast it is quite sensitive. Too much rotation will not open the top and flat the lower part of the sail (which can be fast sailing in flat water and fair winds.
Are the top battens too soft? This is fairly difficult to judge.
Are the top battens too tight tensioned? Try to knot it just in place without any tension for strong wind and see what it does for the sail shape.
Are you having enough downhaul? Tight downhaul means really tight downhaul. Sheet in after you tighten the downhaul and then tighten it again.
Was you sheet too tight?

What was the course and wind strength? Any chop? It is not always fast to open the top in strong winds. If the water is fairly flat, you will point higher or have a better vmg if the sail is flat in the lower part. You have less acceleration but a better top speed, because the boat is less dragy.

Cheers,

Klaus