Hi Alexander,
that's a pretty generic question around CIF and the transfer modes. Therefore I would recommend to read the following articles first to check your specific CIF setup and why the delta transfer does not happen as expected.
CIF - Frequently Asked Questions - Supply Chain Management (SCM) - SCN Wiki
SCM Core Interface- Handbook (PART-1)
Quite some stuff but I am sure this will help you.
rgds, Peter