In monitor mode wireshark will be sniffing packets from other devices. So you have to filter those out or search through them. Also, in monitor mode it cannot be 100% guaranteed that all packets in the air will be captured.
What I would suggest is using a man in the middle attack. In that way all packets transferred is bound to go through your sniffer. Try using ettercap, it comes with kali linux. It will set up a man in the middle topology and then you can start sniffing.