He said the processing system is slow, as the number of vehicles using the automatic cards increases by 20,000 per month.
The number has been rising as the deposit of Bt900 was waived.
"We have received complaints, covering slow refilling service as well as incorrect indication of the remaining amount stored in the cards. We can assure all here all that despite the incorrect indication, the value of money in the cards is correct. The processing system is just slowing down," he said.
The system can accommodate 500,000 trips a day, but now it has to accommodate nearly 700,000 trips.
Chavalit said that after the fixing, the capacity will be raised to a million.
The expressway system now accommodates a total of 1.8 million trips a day.