The gap at the frange is OK as long as the gasket is evenly squeezed.
1. It the pressure still improving? => Your cavity is still out gassing.
2. Fasten the screws little more (not too much) for these two franges.
=> If there is any improvement there is some leakage.
=> If not, this is just a outgas from your cavity.
Do you think your black plastic pieces are vacuum compatible? Are they made of Delrin?