Unternehmen Barbarossa and Fall Blau are two cases I've come across where the Soviets had repeated and reliable warnings about Nazi German military plans, and yet Stalin refused to believe the reports and denied his military the authorization to act on the intelligence.
So... uh... why?
The question here is we know these messages were reliable because we know what happened. While Stalin (or any other member of the Soviet leadership) had no idea what information is correct and had to deal with a stream of conflicting information.
I previously answered the question of similar nature on this subreddit here.
Easy, he had tonnes of reports sometimes every few hours saying attack to come in February then March then April etc etc out of the hundreds of reports westerners like to think that he must have been a knob because He decided to not pay much attention to a report saying 21st June But if I came to you with a tonne of reports all contradictory how to you pick out the correct one?