Have you checked the condition of your bonnet hinge brackets? If not, do so. I will post some photos to show you the wear that occurs over time and causes the bonnet/hood halves to slide forward.
Excess wear suggests that the distance between the scuttle mounted hinge bracket and that of the cowl is too close thus causing the wear. It can suggest that fitment of the cowl and bonnet sides was not done sufficiently well.
The only resolution to this is to ease the cowl forward by loosening the fasteners or by closing the bar that joins the inner wings. Closing will have an effect of lifting the cowl slightly and extending the distance.
Finally, the bonnet catch tension will have little affect upon the bonnet halves sliding; excess wear will.
Beware that any adjustment suggested can have an affect over bonnet fitment and wing to bonnet/hood gaps. It's not difficult but proceed with caution and small adjustments checking constantly.
I hope that this helps.
