Nihalpushpakumara

I have a Nissan Wingroad 2002 (3 meter model) model - automatic. From last couple of weeks onwards, there is a strange issue I am facing. Normally we do travelling around Colombo in heavy traffic and occassionally out stations. While the car running when we slow down or stop the car in traffic light and take off, it will take off but gear will not change. Even when the RPM goes above 3, it will not change. When you stop the car somewhere and restart (shutdown the key and start again) the issue will be OK. Sometimes the car will run continually without any problem. But sometimes this issue will come more than 3 or 4 times in a particular day. So I brought the car to Nilwala motors. They took a scan and told me that there is no any fault showing on the scan. He ran a test round and told that he could not find any fault. But this issue still continuing. I did an oil change. But this issue did not go. Please assist me what I can do next. Thanks......