Without knowing more, a possibility could be the ketone is forming a H-bond with an amino acid, and the o-Br is blocking this H-bond. This assumes that the compounds are acting as typical enzyme inhibitor.

Without more SAR or the specific biological target, this is all guesswork.

For SAR, simple way to prove will be to have groups of varying size in the ortho position. H < CH3? ~ Cl < Br.

Of course changing the substituents affect the electronic properties of the ketone as well, so many factors to consider 

I agree with Eugene Yang...but it will be better if we know about the targeting enzyme. In the enzyme cavity, the ketone is showing more hydrogen bond interaction in case of p-br benzophenone whereas in case of o-br  benzophenone due to setric effect the ketone group is not able to interact. This can come to know by performing molecualr docking study.

Dear Mohammad,

I suggest you take a look on the following links which contain derivatives of benzophenone as anti-cancer agents which may assis in comprehending the differences between the activities of the ortho and para-bromobenophenones:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3614727/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3643672/

http://bmccomplementalternmed.biomedcentral.com/articles/10.1186/s12906-015-0911-1

http://www.tandfonline.com/doi/full/10.1080/00397911.2010.521435?mobileUi=0

http://pubs.acs.org/doi/full/10.1021/om501060f?src=recsys

http://www.google.tl/patents/US8394954

http://researchspace.ukzn.ac.za/handle/10413/12821

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0001-37652015000100289

Hoping this will be helpful,

Rafik

Thank you all for your beneficial answers