Be careful there seem to be people vouching for heads they have not tried personally such as sirui v-a5 now I may have got a bad example but if so the the quality control is rubbish and I would advise against this company altogether the locking system failed and if it not for the fact that I always attach a strap to the foot of my scope and high on the tripod it could have cost me a good scope. By the way if anyone wants a Sirui va5 cheap with all original packaging pm me you have however been warned !!
I have this Head on a Monopod and one day had a slight difficulty getting the AS Plate to locate, opening the lock was not enough to accept the plate in the clamp.
But, after worrying if like yours it was faulty (why, it worked before) i noticed the locking lever was spring loaded ie you could gently pull the plastic lever out and looking with a "light" there was a small shiny metal flange so to speak which when pushed down or up, set the right distance for the jaws to open and lock securely.
You have to keep the plastic lever OUT while gently adjusting this SMALL flange.
The Shiny Flange DOES locate in a recess in the Plastic lever , so when you set it make sure it is all located at the end.
Works a treat now.
Maybe this is all you need to do.
This must of occurred because I must have messed with the lever when the plate was not in and because the lever is mated with the flange it moved out of the right width distance to take the said AS Plate.
hope this helps