I think the problem here is that when the camera looks at the underside of a matrix, the underside is invisible to the camera.
The SET MATRIX command has an option for turning culling on or off (which I believe is the technical term for what I described above).
The below demo shows what I mean. Run it once, move the camera up and down and you see the matrix dispear when the camera goes below it. The unrem the SET MATRIX line (line 8) and run it again and you'll see that the matrix doesn't disapear.
sync on
sync rate 60
`make a matrix
make matrix 1, 1000,1000,10,10
`unrem the below line to see how the set matrix command works
rem set matrix 1, 1, 0, 0, 1, 1, 1, 1
`this variable controls the y position of the camera
ycam = 500
do
`press up and down arrow keys to move he camera up and down
if upkey() = 1 then inc ycam, 5
if downkey() = 1 then dec ycam, 5
`position the camera
position camera 500,ycam,-500
`always point the camera at the centre of the matrix
point camera 500,0,500
`print some stuff
set cursor 0,0
print "us up and down arrow keys to move camera up and down"
sync
loop
I've never used the SET MATRIX command before so I'm not sure not how the values I've used in the other parameters will affect the look of the matrix in your game. You might have to play around with them to get it to look right.