Right, ok, so it’s not the spoke snapping by the sounds of it?
Strange though – I’ve built hundreds of wheels and trued/repaired more and never come across symptomatic nipple breakages – dodgy nipple batch maybe 😯 . Anyway, it sounds pretty clear – get new nipples.
Alloy are fine actually IME though I use brass because I don’t think the cost of alloy is worth it. If built well and they’re not corroded, I’ve had no problem rebuilding or truing with alloy nips.
Just one further thing – where is the nipple snapping? Is it leaving its head in the rim?