Jose,
Dirt in the light trap grove in the camera back will prevent the film holder ridge from seating fully, common problem.
Worn film holder ridge can slip out of the back grove easily, rare problem.
Standard light leak detection on a large format camera:
Put a 40 watt appliance light bulb in a portable lamp. Appliance light bulbs are clear and get very hot. A small flashlight that can be adjusted to spot at 1 to 2 inches can be used as well as CFL's and small fluorescent lamps.
Extend the bellows fully, remove the lens board, insert an empty film holder in the back and remove the darkslide.
In Total Darkness turn the light on and insert it into the bellows.
Observe the bellows from the outside for any hint of light, pin holes will be white, weak spots in the bellows will be yellowish. Move the light slowly from one end of the bellows to the other.
Observe the back and darkslide slot of the film holder.
Install the lens board and remove the camera back or focus panel.
In total darkness turn on the light and insert into the bellows from the rear.
Inspect the front standard and lens mount for leaks.

Carbon black artist paint applied to the inside of the bellows works well for pin holes and weak spots.
Tears can be repaired by gluing new material to the inside of the bellows.
Leaks at the lens board or camera back will require repair to the camera, lens board, or film holder.