Diagnosis
PartyBoost is finicky about sequence and existing connections. You cannot mix with older JBL Connect+ speakers.
How to Fix It
On the *first* speaker, press the PartyBoost button once to make it the host. On the *second* speaker (the Flip 6), press its PartyBoost button once within 30 seconds. Wait for the tone confirmation.
If That Didn't Work
Power cycle both speakers. Ensure both are updated to the latest firmware via the JBL Portable app. The Charge 5 can only PartyBoost with other PartyBoost-compatible models (Flip 6, Pulse 5, newer Xtreme 3).
Heads Up
PartyBoost uses a separate, dedicated wireless protocol, not Bluetooth. Keep speakers within 3 feet for initial pairing. Stereo mode (left/right) is only possible with two of the *exact same* model.