fix wall error

Dear friends,
    I used fix wall command to set a flat boundary on the higher z axis. My command is,
    fix wall water wall/lj126 zhi 290.0 0.0803 3.5 12.0 units box pbc yes
    The error is particle on or inside the wall surface.
    But when I looked up the trajectory file in VMD, I found that the z axis position of highest atom in the z axis is only around 79. It was far away from the position 290 I set. The box is a 300 angstrom cube. I have left 10 angstrom as the manual told when using pbc.
    So what is wrong here? As I see it, there is no atom on or inside the wall.

there is not enough tangible information here to make any specific statement.