Problem with unlocking and locking the Materials in MARA using idocs.

We are sending a inbound idoc from one cluster to the other to update the materials

If the material is locked ( MARA-MATFI=x ) before it needs to be unlock it ,update it and lock again .

If its not locked ,we just need to update it and lock the material .

When we are trying to implement this the materials are not getting locked at all times .

the update inbound idoc is being sent to 5 clusters at a time .

The same material is getting locked in some clusters and some it is not . (But the data is being updated in all the clusters )

Can you please suggest me a solution or give me an idea where the problem would be lying.