Rail joiner as they come with the rails from the manufactures are good for about a year outdoors. After that the contact becomes questionable. Oxidization and dirt are making the contact resistive and eventually they become intermittent and fail completely. Conductive grease is not really very conductive, but it helps, not in making the contact, but by keeping the dust, dirt and soil out of the joint. Lets not forget that exposed to the weather the rails are in constant motion due to the thermal heating and cooling. It causes the dirt to enter deeper and deeper into the contact areas. On my fist layout I soldered jumper wires over each joint with good success, but the soldering was a main chore considering the many joints. A friend had an idea. He drilled a small hole at a 60 degree angle trough the rail joiner, the foot of the rail and out again through the bottom of the joiner. A small (#4, 3/8 inch long) stainless steel self-tapping screw is placed in this hole. Two for each joiner, four to each track joint. It seem a lot of work, but it is still less of an effort than to remove each joiner and replace it with a much costlier clamp. A little touch-up with paint makes the screw heads almost in visible.
My present extensive layout was build following my friends idea. It has close to 500 track joints (2000 screws) and not one has failed to conduct in 17 years !!!
Ride the rails in one of my videos and see if you can spot them: