The party is at level 6, but what level are the enemies at? The level gap is very meaningful in PF2, with PL+2 creatures being twice as powerful as a PC. And hitting someone that's twice as powerful as you is, well, you know... Hard to do.
Are they fighting lower level enemies and still failing to do real damage? What's their gear like? And are they using teamwork? If they're not trying to impose debuffs on enemies, I would adjust how I fight, and introduce more ad-hoc circumstance bonuses and penalties.
