Like others said the point of the combiner is that all information on the HUD always appears over the same part of the background irrespective of where you move your head. That being said you can't really move your head very far before you lose sight of the information completely.
Having recently moved to an aircraft with a HUD my major gripe is that there is distortion where the combiner glass ends. Now many people have told me to sit closer to the HUD but the issue is is that that would bring me way to close to the glass. Moving to this new aircraft has already completely changed my seating position due to the HUD and I think the distance required between the pilots eyes and the HUD is not designed realistically by the manufacturers. The only solution in my opinion is to increase the size of the combiner glass by around 50℅ in order to cover more space and allow the pilot to sit in a more natural position.