Regarding field is not populating automatically for Activities automatically in CRM2013?

Hi,

I have created one subgrid for appointment in contact entity. When i click on new Appointment in subgrid, one new appointment record is opening, but the related contact is not displaying in Regarding field of appointment.. This problem is coming for CRM2013.

In CRM2011, it is working perfectly. Please let me know what is the problem in CRM2013.

Thanks,

Sravan.J