There are a couple of ways you may improve your signal. Both however depend on how far away from a cellsite you are.
1. You could try mounting the booster aerial outside. Extension coax (pre-terminated) is available from a Woosh regional office. If they don't have it, they may be able to get it for you. Recommend two lengths (5m each) and the modem tail only otherwise you'll suffer too much cable loss and be back to square one.
2. See if it is possible to get a gridpak aerial. Although this is costly and needs to be installed by a Woosh approved installer/contractor, you should get a major increase. But expect to pay about $350+. You'll find that they won't install one of these if you are within the non-extended cell range of 6.4kms.
Also worth trying the black booster rather than the white one, and making sure that your woosh modem is on the right side of your house to where the cellsite is. Woosh have now put up a google maps plugin that shows you where your nearest site is, or you could check http://wooshinfo.googlepages.com/locations.html which is a site I built out of the Resource management spectrum database listing all the transmitting locations for all transmissions within NZ. I cut out the woosh specific ones and put them into google maps.