Transmission slow to shift

astro van with automatic transmission when starting off it doesn’t want to shift on its own. i have to manipulate it to shift with the accelorator. at about 22mph i let off the accelorator slightly and it shifts to the next gear

Model year?
Engine type/size?
Odometer mileage?
Maintenance history of the transmission?

Does the problem clear up once the engine is at operating temperature?