I've also had the same issue. The Old Victrix Republican Romans have thicker, better pila and long spears, and they're unlikely to break compared to say Carthage, numidians, or other sets they do with the skinny spears.
I've read and now tried coating the spears in PVA glue and it seems to slightly help. IT dries clear so when you get to priming it should easily be covered. So far I've had some success doing that, but it does not make the spears invincible. They're still not as good as thicker plastic spears from the old republicans.