1) For immediate Pirates use the press-gang edict on unskilled captives. Best possible pick are captives that are unemployed, high courage, close to town, really tired & hungry and have learned few if any skills. Don't press-gang a hauler carrying a load of goods!
But I'm also running low on captives too. Sure I got 140+, but they can't service everything.
2) You have 'Board-em' settings. Bit of a two edged sword that. Risk to your existing Pirates but supposed to capture more. Try setting 'Big Spender' as well. Cuts the cash your island earns but supposed to sway battles in your favor as enemy surrender easier to join your cause.
On a few Board em' missions I've had all my pirates come home, even in areas with heavy naval activity. I always use big spender because it draws in the new pirates. But I still have problems.
3) Try using smaller ships. This statement might be contested by others. My theory is that big ships have a fearsome broadside. All battles start with one round of this. There are many encounters against smaller ships. Against your Frigate the few it catches are pulverized with few survivors. A smaller ship with a high notoriety Captain will rely on cowering the enemy into submission. I suspect your Frigate has many KIAs also which need replacements.
I've actually had a lot of new pirates come in when I used a Snow (yes, a Snow) with full cannons, muskets, and cutlasses. The little Snow went out on a Pound em' mission, engaged in one battle, and came back with 15+ new pirates.
And they say Snows are too weak to cruise. Fah!