LED bulb always flickers or doesn't turn on
The most common cause of this is the 12V transformer. A GU5.3 bulb runs at 12V DC instead of normal line voltage (110-230V AC). Many transformers have a minimum load for normal operation, which the LED bulbs may not be drawing.

As an alternative, you may want to consider installing GU10 bulbs. They are the same general size and shape as GU5.3 bulbs so many fixtures are compatible, but they operate on line voltage so there is no longer a need for the transformer. You would just need to remove the transformer and replace the socket, which is a very small cost on top of the bulbs. It is the best way to negate the flickering and troubleshooting headaches.