Automatic gear not shifting properly

Hello,

We bought recently this model (May 2020) with a 1.4 VTI best collection, which has little km for its age (around 61 000 km) and we have this issue which arrives not often but still persisting: when from small speed to higher (quick acceleration, small gear to higher) accelerator does not respond properly and it needs several seconds to accelerate. We recently changed the fluids and technician checked the gear and told us there is no problem but it still arrives sometimes in situation when you need to accelerate quickly when waiting the lights or from not priority road/stop sign and you need to go quickly. I was thinking that the fluid can be an issue but after our technician checked it I do not know if it is us who did the acceleration wrongly or else. Thank you in advance for your help and advise.