It is not usually possible for the University to act as the immigration sponsor for a repeat period of study.
The University is required to have strong justification for offering further immigration sponsorship for a resit period, which is why it is able to do so only in exceptional circumstances, where attendance is academically necessary (i.e. there is no option to resit assessments as an external candidate), attendance has been good on the course to date and the student has EC evidence.
The student must also be able to complete the course within UKVI time limits for studying in the UK.  
This is confirmed in our Limitations to Immigration Sponsorship policy.