Meetings are being held on a daily basis to try and resolve the issues with Dublin’s Luas Green Line, transport officials have said.
Commuters have complained of longer wait times and a slower service since the Luas cross-city extension began operating in December.
The National Transport Authority (NTA) has blamed works taking place in the Sandyford depot, and it has admitted that there are problems with the longer Luas trams.
NTA CEO Ann Graham has apologised unreservedly to commuters.
She confirmed a fault has been discovered with the new longer trams being tested, and therefore they have been withdrawn from service.
She noted every effort is being made to identify the fault and bring the longer trams back into service.
— Juliette Gash (@JulietteGash) March 16, 2018
She said: "All parties are involved in the provision of services... but the NTA, Transport Infrastructure Ireland, [Luas operator] Transdev and [transport company] Alstom are meeting on a daily basis now to address the issues associated with the provision of the Luas Green Line services.
"The main issue has been the availability of fleet, but we would recognise that it has been a suboptimal service."
She pledged that there will be an improvement in service by May once all longer trams are in service - but stressed they want to make sure as many trams as possible are operating before that.
In a statement, Luas operators Transdev said there has been some reduction in services on the both Luas lines since the opening of the extension.
They explained: "This is due to availability of trams and high passenger numbers at peak times. We will continue to review the service and make any minor changes we can to improve the service.
"However, you will see a noticeable change in service when seven new 55m trams come into service over the next few months. Thank you for your patience and continued support at this time."
The Luas extension has also caused traffic problems in the College Green area of the city, and the NTA has previously announced a number of measures to deal with that issue - including diverting some bus routes and restricting taxis from the area at peak-time.