Translink have said they do not know when their mobile payments app will be fixed.
Customers took to social media to vent their frustration at the difficulties with the mLink app after experiencing issues for three consecutive days.
The company says the problem with the mLink is a UK-wide technology glitch and not directly related to the start of the new Glider buses in Belfast.
Translink ticketing manager Simon Hunter apologised to customers.
He told the BBC the technical problems were mainly at peak times, and insisted that customers double-charged for tickets would receive refunds without having to fill out any forms.
The mLink app allows passengers to use pre-paid tickets on public transport.
@Translink_NI what’s going on with the translink mlink app.. can’t seem to buy a ticket app keeps timing out??— William (@WilliamDillon85) September 5, 2018
William there's an issue with mLink, supplier working to resolve ASAP. Apologies^CD— Translink (@Translink_NI) September 5, 2018
@Translink_NI 3rd day running MLink not working and can’t now access £40 monthly ticket bought for daughter school travel when came on line last night. Having to pay cash each day. Can’t access confirmation email as proof of purchase.— Hendy (@thehendana) September 5, 2018
Earlier this week, Translink said the problem was a result of "supplier capacity issues" which affected operators in the UK and parts of Europe.
In a statement, Translink acknowledged the purchase of new tickets "was not possible in some cases".
They have urged customers who are experiencing problems to contact them.
The issues with the app the past week or so seem to magically happen in the morning, when you have more passengers, and is always fixed in the afternoon....— Rosie Hamilton (@slimemonkey) September 5, 2018