Bustercrabbe

Could the main drain in the main pool be clogged??

I bought an existing home with an in ground concrete spa, which spills into pool. Has six jets along the sides, and a main. Three of the jets “feed” water/flow fine. Three of the other jets suck water. Main is working, but hardly. Can anyone shed some light on what could be wrong? Also, someone unscrewed (missing) the “eyeballs” on all the jets. Am pretty sure they are Venturi type jets. Any help would be greatly appreciated and thank you for your time to reply. Buster