i had the same problem with my usb mouse on my laptop. sometimes the middle mouse button would engage and sometimes it wouldnt. usually it wouldnt.
there is an alternate way of middle clicking on a laptop. here is what i did:
open control panel and open the mouse and pointers icon (i think thats what its called on laptops). i think under the TAPPING tab, you are able to configure what buttons do what on your touchpad. leave the left and mouse buttons on default. there is an option to set the right and left buttons to middle click when simultaneously selected. enable this option and youre ready to go.
its tricky to pan/move (i dont know what its called) a camera with this setup. with a normal mouse, you would simply hit alt, middle mouse and drag move as youd like. to do this with this setup, youll need two hands. with your left hand, youll need to hold alt and both right and left mouse buttons. with your right hand youll have to actually move by dragging on the touchpad. its not so hard once youve done it a few times. you get good at it pretty quick.