The SMFA will block any transfer that they deem unusual or dubious. Their decision is aided by our Automated Transfer Monitoring System (ATMS) and is not personal or a judgement on you. The deal could have been blocked for a variety of reasons such as not being fair, too heavily weighted towards one club or issues with one or more of the managers accounts. If you feel the deal is fair and there are no issues with your account then the problem may be with the other managers account which the SMFA cannot discuss.
The ATMS 'learns' over time about managers and how fairly they are playing the game. Managers that continually break the rules will find it harder and harder to deal with other managers and more and more of their transfers will be blocked.
If the SMFA have blocked you from conducting transfers with another manager, they will not reverse their decision and it is advisable to look elsewhere to conduct your transfers.