Hexapod question

I've started building a hexapod on the cheap. Some photos:
http://www.tinyplanet.com/images/hexapod-portrait-reduced.jpg
http://www.tinyplanet.com/images/dscf0011-reduced.jpg

I noticed that the platform can be rotated and tilted more or less
freely, even with power going to the steppers. Translational movement is
a lot harder. This makes me think that I misunderstood completely how a
hexapod is supposed to work. I have a universal joint on the frame so
that the motor can tilt, and another on the platform.

Can this setup be made to work? Why doesn't my hexapod lock the platform
in place?