Tank valve
The tank valve and inlet filter
Rocket's manual says not to attempt repairs yourself: contact an authorised, qualified technician, and any repair must be done by Rocket or an authorised dealer using only original parts. Rocket does not cover damage from unauthorised changes or non-original parts. Treat this as dealer work.
The tank has a tap in its base, opened by a peg in the bay when the tank is pushed fully down. The manual says press it firmly down so the tap fully engages. A tank not fully down, or with a warped base, starves the pump (an owner of an older Giotto: "just had to do a pretty firm push downwards").
Between tank and pump is an inlet filter (C439900695 on the FAST sheet), which owners of the older Giotto found clogged when the pump was slow to build pressure. Reaching it is technician work.