Auto box late shifting between 2nd and 3rd

Hi all. Having an issue with my auto gearbox.

The van is taking ages to shift between 2nd and 3rd gears. In some cases (on the flat), I can get to nearly 70kph before the box shifts up.

I can prompt it to change (when I think it should change) by easing the gas pedal.

No issues between any of the other gears, going in/out of overdrive, or dropping down into 2nd from 3rd.

Any suggestions on what might be going on? Dodgy solenoid perhaps?

Thanks folks.

What year, engine, efi?
And are you running synthetic or mineral trans fluid? When’s the last time you changed the fluid?