The day your lighting system is installed, Mother Nature starts attacking it. Water seeps into low quality fixtures and rots the sockets which causes wire corrosion. Insects can wriggle into fixtures and nest there if they’re not properly sealed — these tiny pests will even fill fixtures with so much dirt, it completely blocks the light output.
If some of your lights are going out, you may have connection problems.
How are your fixtures connected to the wiring that runs to the power source?
Because your system has wire nuts & connectors that aren’t designed to go underground, chances are they corroded, so that’s why your lights aren’t working. There’s no repairing these — they should be replaced with watertight, heat-shrink connectors that will resist corroding when underground.
The transformer is the power center that powers your entire landscape lighting system.
If your lights aren’t working right, they may have a loose connection inside the transformer. Over time, as your lights are turned on and off, on and off, the electrical connections get loose.
Sometimes, loose connections can cause “arcing,” when electricity jumps across a loose connection, creating heat. This heat can melt connections, causing shorts or even fire. If that’s the case, there’s often no fixing it — the transformer may need to be replaced. And they might be replaced with an MV/LV oil transformer instead, this is seen as more environmentally friendly by using vegetable oil if opted for it and can be safer in the event of a fire, this link http://www.gbeuk.co.uk/products/mvlv-oil-transformers/ talks about the benefit of the transformers.