I suspect one of your splices is shorted, this causes excessive
current from the amplifier, and the protection circuitry cuts in,
turning off the output.

Given the reported symptom, that's what I'd expect.

Home Depot has a 100ft roll of 12 gauge for twenty-something
dollars, just buy it and cut it in half to get two speaker runs of 50
feet. Or if the distance is actually more than 50 feet, buy two rolls,
or even the jumbo 500 feet roll and save more per foot!

Or just buy however many feet you do need off their big roll.

There's nothing inherently wrong with a properly done splice. But wire's
so darned cheap that there's rarely any good reason to splice for
anything but the most temporary solutions.