My character is falling through the floor

Well i made a level out of boxes that worked ok, but then a made some bits in edit made, and that was trouble, i made a few chunks of floor and joined them together, but the polygons i joined with seem to not be detected as stable floor.

First check in edit mode that all floor polygons have normals pointed upwards (check “Draw Normals” in edit mode, editing buttons (F9)). If there are any faces that point in wrong direction, select them and press W and choose “Flip Normals”. If that didn’t help check in face mode that those faces have collision set on (press F, select floor faces, press F9, press “Collision” if it isn’t already on, then press “Copy Drawmode” and exit face mode). You should also set the level bounds as “Static TriangleMesh” in logic buttons (F4).

thank you for the advice this worked for several bits of wall and floor
, but then when I pressed copy draw mode half the polygons seem to have turned invisble in the game.

Are any normals pointing away from the camera?

If they are, but you need them that way, toggle the ‘twoside’ button