EXCLUSIVE

ScHoolboy Q was under investigation for felony armed robbery, but will not be charged due to a lack of evidence against him ... or even witnesses accusing him.

It's a weird incident no one knew about, but our law enforcement sources say it went down back in March after an L.A. concert featuring ScHoolboy Q,

The alleged victims said they followed Q to an after party, and when they arrived 2 people robbed them at gunpoint. They admit the rapper "never participated in the actual robbery," but told police he showed up in another car while it was happening. Their insinuation was Q masterminded the jacking.

However, we're told the alleged victims delayed reporting the robbery at all, and when cops reached out ... they had no interest in filing charges. All of which explains why the D.A. just officially rejected the case.