Opayo / SagePay 3D Secure integration

I have a client who's e-commerce website takes payments with SagePay / Opayo, we have currently implemented 3D Secure when adding a card to the system. The system uses Tokens to take regluar payments from clients. These regular payments are then not being Authorised.

Some transactions which are being put through are authorising OK, but 70% of payments are being declined for some unknown reason.

I am looking for someone with experience in E-Commerce and especially Opayo who can take a look. The website is a custom PHP site.