WPA/WPA2 kali linux

I have recently started to use kali linux wanting to know more about testing. I started out using it for WPA/WPA2 cracking test. I tried to attain WPA handshake but no matter how many times I do

aireplay-ng -0 2 -a mywifibssid -c my_mobile_bssid mon0

It's not letting me get the handshake. What is the possible problem that's causing this? Thank you in advance!