can you elaborate what network setup you are currently running?
just setting the network card to monitor the traffic passing by is not enough. the traffic needs to be actually 'passing by'. your monitoring network card needs to be on the same 'collision domain' as the cards that you are trying to monitor.
You are on the same collision domain if you share the same layer 1 (osi-model). Just connecting the card to an etherent port on the router does not work. This is when:
all the cards are connected to the same wireless ssid on the same frequency (=>the same ssid on a different access point does not count)
all the clients are connected to the same hub (not 'switch' as switches are layer 2 devices)
wireshark can also pick up traffic that is passing through the machine. (e.g.: the programm wireshark is actually running on the router or any other device where the traffic is passing through.)
if you happened to have a 'managed switch' (layer 3 device) you can also see if it supports a function that lets you mirror all the traffic to one specific port. if you are lucky you might as have the option in your router to set a costume 'Gateway', which should allow you to redirect all the traffic to another machine, which then sends a copy to wireshark and forwards it to the proper 'Gateway'
EDIT: while you can get the traffic using wireshark. you will only see encrypted traffic since most sites (ie youtube) use https.