Gosh - how often trapping becomes all about patience.
First off - do you have a garage or another place you can put the ones who've already been trapped? They can stay there for a few hours (or a day or so) until mama is trapped.
If not, here's a trick. Loop a length of twine through the door of the trap (in the little hole where the hook goes that holds it open when set). Tie the twine in a circle and hold the trap open, while you put a pen through the twine, thus holding the door open. Tie a long length of twine to the other end of the pen. Take that twine and walk away from the trap - far enough so you're not spooking the cats but close enough to see.
Whenever somebody you don't want to trap goes in, leave them alone. But when someone you do want to trap goes in, you can yank on the twine, pull the pen out of the circle and - BAM! - shut the door.
You may have to keep rebaiting the trap but eventually, you'll get the one you want.
Remember, those cats are alive because they are smart and savy, so it might take a human some time to outsmart them. But you can do it - I know you can.