The management at Kew aims to have the lift operational by early October.
The 18m-high structure was designed by Marks Barfield Architects to allow visitors to get up close to wildlife in the canopy.
But the lift, which has cost £200,000 so far, is still considered too dangerous to use.
A Kew representative said: "The lift is still being commissioned by the architects and has not been officially handed over to Kew. The architects are waiting for the structure to settle."
She added: "This is a very complex structure and it is inevitable that there will be movement in the pylons and walkway.
"We must ensure before putting the lift in operation that the lift is fully commissioned, calibrated and has been tested to high standards."
She continued: "It was too tight a schedule to install the lift and fully commission it prior to the opening (because of) delays with the design of the lift facility and the complex build of the walkway."
A representative from Marks Barfield Architects said: "We are continuing to help Kew with any outstanding issues."
Until the lift is in operation, disabled visitors are being told it is not working before they pay their admission fee.