Problem
Valve is learning and it is possibly on the outflow side of the radiator, because of this it is taking a long time to react.
Solution
The reason for a valve's slow reaction time in the first week is that there are 500 steps in the electronic valve from the fully closed position to fully open. This makes it extremely accurate and quiet when controlling to a set point temperature in normal operation.
When a valve is first mounted on a radiator it has to learn the 'on' position, so it can work efficiently and react faster in the future. When it is in this 'learning mode', each time the valve is called to heat a room it opens 5 steps, waits to see if the temperature of the hot water flowing through the valve body (the metal part that the water flows through) changes, and then after another 60 seconds it opens another 5 steps etc. When mounted on the flow side of the radiator it will normally have established the 'on' position of the valve after about 15-20 minutes, it will store this in its memory and will then fully open as usual to allow the radiator to warm up.
If it is mounted on the out-flow side of the radiator it may reach the 'on' position, but because the hot water is flowing into the other end of the radiator the valve will not sense a temperature change, it will continue on the 5 steps routine for much longer. After the first week the valve will have eventually worked out the correct 'on' position but it will take much longer for it to do this in this case.
To work out what side of the radiator is the in-flow and what is the out-flow you can purchase magnetic radiator thermometers, or you can simply use your hands (WARNING: the radiators may be very hot to touch). The warmer end of the radiator nearest the valve/pipe is the in-flow end and the cooler end of the radiator is the out-flow end.
You can stop a valve from learning if you simply press any button on the valve whilst it is trying to warm up the room. In this case the valve abandons the learning for that time and will wait until the next time it is called to heat a room before it tries to learn again.