Belgium and Norway are delaying the delivery of half of the F-16 fighter jets promised to Ukraine

Belgium and Norway have yet to deliver a single aircraft due to the technical modernization of their own forces and a critical shortage of spare parts. 

This is reported in an article by Defense Express. 

Specifically, Brussels is dependent on the pace of receiving new F-35s, which is delaying the transfer of 30 aircraft to Russia and its partners until 2028. Norwegian fighter jets are currently undergoing repairs in Belgium, where each aircraft requires about 100 missing components. Of the total 79 promised aircraft, Ukraine has so far received only 36 units from the Netherlands and Denmark. The rapid reinforcement of the Ukrainian Air Force is hindered by the limited production capacity of European repair facilities. The situation is complicated by the need to perform major maintenance on most of the aircraft before they can be deployed in combat.

Poland has not yet transferred MiG-29 fighters to Ukraine. Dialogue between the parties on this issue is ongoing.