Left turn signal better than right

The solder-blobs on the base of 1157s sometimes simply get dimples from the compression of the button contacts.
Sometimes you can give them a dressing with the edge of a screwdriver.
You can get out the soldering pencil and build it back up.
You can simply put in a new bulb.
See if the brass buttons in the socket are still shiny....
Maybe the socket has poor grounding.