Deadline to execute oil & gas leases

Last year I signed two new leases in Williams and Mountrail counties, ND. My brother also had parcels for the same locations, however, he passed before getting the lease offers. It’s been about a year now and his mineral ownership was recently transferred to me. I have emailed and left VMs to the two landmen that facilitated the leases. Neither one has responded so I’m wondering if I’ve missed a deadline. I’d like to received the bonuses and royalties but am I out of luck? What should I do next?