Resolution:
Check that you have not given a very small value for file size (option "&ltsizeLimit>"  in fileconfig="&ltfileName>^&ltsizeLimit>^&ltnumFiles>^&ltappend Data>" ) .

For example if you have given something like this:

mbengine -repourl tibcr://TAFOCOBAFI026:service=2211:network=;239.192.1.21:daemon=tcp:2211 -transformation MBAFOCOBAFI026  -file true -fileconfig "c:\tibco\messagebroker\log\john.txt^5^2^true" -engine MBAFOCOBAFI026

This won't work - noting will be written to log file john.txt since you have given file size to be 5 bytes that is such a small size of file to be written anything  .

So, provide big number something like "2000000" ==> 2MB .

Bottom line is option "sizeLimit" is in Bytes not in KB or MB .