Hi Mike,
Since BP replication of master data is a BDC( screen based ) approach from CRM to ECC, hence there could me some rare cases where the screen fails and no error etc. is shown in the BDoc.
I can suggest you a few things and see if any of these steps correct the whole process.
1. Try sending the BP from CRM using tcode : BUPA_SEND.
2. Download all details of customer from ECC again using CRM tcode : CRMM_BUPA_MAP
3. Check for dumps in ECC
4. Check for update fails in tcode : SM13 in ECC
5. If none of this helps then you may have to debug the outbound RFC queue for this BP.
/Hasan