From what it looks like in your question you aren't deauthorizing any users off the network. So that means the only way you would intercept a handshake is if someone joined the network on their own (Like just walked into wifi range or change to that wifi). If you deauth people from their own network they might notice, but the most people might just think it was their wifi being crappy.
To deauth people using Aircrack the code looks something like this.
aireplay-ng --deauth 5 -a (enter BSSID of AP here) -c (enter MAC targeted client here) wlan0
5 is the number of deauthorization packets it sends out, you can put more if you aren't sure it's been actually been booted off the network.
This link should help you with any other questions you might have about deauthing people with Aircrack
Another issue could be the distance, if you are too far from the device itself you could have just missed the handshake all together.