The capture HOs are almost always verified : the MS in the umbrella or in the GSM900 cell is probably always getting good signal from neighbour cells. But the HO is not trigger as long as some extra criterion are not verifies.
for cause 14 : umbrella traffic and min dwell time.
for cause 21 : serving cell and target cell traffic load.
what you could do is ease up those criterion, so that MS would safely goes to the neighbour cell, rather than waiting for the drop in the serving cell.