My nose is always slightly blocked up, clearing only when I sneeze, but I have been to an allergist and have no allergies. What can cause this?

Nonallergic rhinitis. About 15-20% of the population are born with the genetic tendency to develop inflammation in their nasal passages, i. E. Rhinitis. Of this group 80% produce allergic antibodies (ige) which leads to allergic reactions, further inflaming the nose, but the remaining 20% have nasal inflammation without an allergic component. Your allergist should be able to treat your non-allergic rhinitis.
Possible anatomical . There can be an anatomical problem such as a polyp, bone spur or deviated septum. There can be non- allergic sensitivities from irritants, medications, environmental conditions. You may benefit from an ENT evaluation. Your allergist is also equipped to diagnose and treat many non- allergic causes of congestion.