[AndreT]

You cannot instantly attach the trailer. The trailer must first stream in for the player, then AttachTrailerToVehicle can be used.

Here's how I (and I assume, most people) bypass the issue:

pawn Code:

// Where you create it
SetTimerEx("attachTrailer", 1000, false, "ii", createdvehicle, vehicleid);

// The timer
forward attachTrailer(trailerid, vehicleid);
public attachTrailer(trailerid, vehicleid)
    return AttachTrailerToVehicle(trailerid, vehicleid);