The setback means the trains are now due to carry their first passengers two years late.
They are scheduled to initially operate with drivers before becoming the first in the UK to run without any staff on board.
The trains will be able to operate more frequently than currently, with open-plan walk-through carriages rather the current separate ones.
Strathclyde Partnership for Transport (SPT), which runs the 15-station circular underground railway, blamed the contractors for not getting underway on time.
The Ansta consortium of Italian signalling firm Ansaldo and Swiss train builders Stadler won the £200 million contract as part of a £288.7m overall Subway upgrade, which also includes stations being overhauled.
SPT chief executive Gordon Maclennan told an SPT meeting today: "The real problem with this contract is they started too late.
"The main bone of contention was because they used up a considerable amount of float [slack] at the beginning, telling us this will be fine, ‘It's well within the float'.
"I don't think they themselves understood how complex the project was going to be."
Mr Maclennan said having Swiss, Italian and French parts of the consortium had also caused difficulties.
He said: "We have had significant meetings with them and the way things are going at the moment, I think we have more assuredness in their project timescales than we've ever had.
"It's an improving relationship, but still contractual and at times confrontational."
SPT director of Subway Antony Smith said: “We expect to see new trains on the system in 2022 and that’s when our passengers will really appreciate the new fleet.”
SPT had said when it awarded the contract to Ansta four years ago: “The public should start to see the first new trains introduced to the system in 2020.”
Work was halted by the Covid pandemic between March and June, but an estimated six months is expected to be saved by introducing the new trains using the existing Subway signalling while a new system is installed.
The first three of the 17 trains are being trialled on a surface test track south from the Subway depot in Govan.
They are due to be tested in the Subway next year overnight when the system is closed.
The trains will operate with drivers until platform screen doors are fitted at stations, similar to those on some London Underground lines.
They will then switch to “unattended train operation” like some underground lines in Paris, Barcelona and Copenhagen.
Other UK metro systems without drivers, such as the Docklands Light Railway in London, still have on-board staff to operate the doors.
SPT said installing the platform doors would be the last stage of the project.
A spokesperson for the consortium said: “Jointly, Stadler and Hitachi [which owns Ansaldo] are working hard with SPT to deliver the first full upgrade of the Glasgow Subway in more than 30 years.
"The project is made more complex by the unusually narrow tunnels and track in one of the world’s oldest underground networks, but once complete it will offer passengers a vastly improved service and an easier, more comfortable way to get around the city.”