Sticking drivers door Gransport

[Hedgetrimmer]

I seem to have an intermitent problem with my drivers door. Regularly it won't open from the outside even though I can hear the central locking working. The passenger door still works and I can just get the drivers door open by using the internal drivers door handle. It still takes a little effort to open from the inside.

Can anyone shed any light on what the issue might be?