Sources connected to Bynes tell us ... all 3 people are telling similar stories -- Bynes was a dream client for years ... but she became extremely difficult in the past 12 months, some say uncontrollable.

One source tells us the management team hasn't been able to get in touch with Bynes for the past month -- no phone calls, no visits, nada.

According to one source, all 3 members of the team made multiple efforts to help Amanda before she went radio silent -- but Bynes was not receptive.

Eventually, the team decided to cut bait ... acknowledging they can't help someone that doesn't want to be helped.

As for Amanda, sources close to the 26-year-old tell us ... Bynes feels she no longer needs their services because she's retired from acting and moving into another line of work -- fashion.