Debuffs will rarely be outright reliable, they'd be broken if they did considering how much they swing the turn economy.

That said, boosting spell save DC goes a long way (either by primary stat or gear) and gets pretty strong by late game. Upcasting gives you more targets to try your luck on, too. Early game you're bound to have some misses just like your martial fighters do, but Sleep does very well then, as does Phalar Aluve: Shriek.

In general, having options that check against different saves and being smart about your targets helps a lot. Don't try to throw Hold Person on an enemy druid or cleric, and use Bane on low CHA targets like undead and constructs, for example.