Why do the mines I lay down not go off when walked or driven on?

The mines in OFP are anti-vehicle mines, for use against heavy vehicles, such as APCs and tanks. Trucks, jeeps and cars will not trigger OFP's mines.

Currently, OFP does not have any AP (anti-personnel) mines. However, these can be simulated using scripts available at OFPEC. Even better, the Chain of Command (CoC) Mod has an excellent mine addon that is chock full of AP mines. So step on it and download their CoC Mines addon now!