Hi Motit,
What you have done is absolutely correct. There can be two chances not reflecting updated material group in BEx report.
Case 1: Your BEx query might be running from Cache memory, just delete cache and re-run your query.
Case 2: If you are physically storing material group at transaction data(InfoCube or DSO) Then changes to master data will not reflect, You have to choose navigational attribute then it should work.
Thanks,
Kuldeep