Law enforcement sources tell TMZ ... the investigation into DaBaby's bowling alley brawl with DaniLeigh's brother, Brandon Bills, has hit an unexpected roadblock because the alleged victim is no longer cooperating with police.

We're told, even though much of the fight was caught on camera, it's very difficult to move forward without a victim.

As we showed you, DaBaby and his crew got into a massive fight with Brandon at a CA bowling alley, back in February. Cops were on the case, after video footage seemingly showed the "Suge" rapper throwing the first punch ... before his crew gets involved.

After reviewing the footage, police reportedly opened an investigation into the rapper.

ICYMI, beef between DaBaby and Brandon escalated ... after Brandon's sister, DaniLeigh -- DaBaby's baby mama -- publicly aired some issues. At one point, DaBaby and DaniLeigh got into a blowout fight and DaniLeigh was arrested for assault.

We're told the bowling alley investigation remains active but isn't going anywhere at the moment ... because Bills isn't working with police.

No word on why Bills isn't cooperating ... but of course, he could change his mind.

Last week, DaBaby shot a trespasser on his North Carolina property. In the 911 audio following the shooting, DaBaby talked back to the dispatcher and told them he'd refuse to put his gun down when cops arrived.