Are you having a little trouble getting the hose end into the SUP board's valve?
To connect the end of the pump into the valve, you do need to push down before turning to catch the hooks. The gasket on the end of the pump's hose needs to be large enough to ensure a tight seal with the board for easy inflation, and this means you need a fair amount of pressure to hook the hose to the valve. Also, if your pump came with an extra gasket, switching to a different size gasket may form a better seal.
Here is a video that may help:
Hope this helps!