Could be your 1/4 jack on the mixer is dirty or the adapter you are using could be dirty. Get some contact cleaner from Radio Shack, open up the mixer (power off and unplugged), find the jack contacts and spray some on it, then use your adapter to plug in & out of it quickly a few times to work the stuff in. Let it air dry for a few minutes before you test it.
This will work for the 1/8" headphone jack too, if you can get the spray down in the jack or know how to open up the laptop (don't try if you are unsure). Again be sure the power is off and let the spray evaporate before you turn it back on. If you just spray down into the jack, leave power off for a good half hour. You don't want to short out something!
1/4" jacks like this use a mechanical movement to open and close contacts for the connection, where RCA and XLR are straight in contacts with no movement involved. That is why you get a better connection through them. I won't even use jacks for my microphone sets if I have another option like XLR.
