It is always the case that ACL surgery is done with an eye toward restoring all the damaged structures to as normal as possible. That way, the patient has a fighting chance of returning to normal function with a stable knee. But it is also agreed that the anatomy of the anterior cruciate ligament is complex and difficult to mimic.
One of the more difficult aspects of ACL reconstructive surgery has always been drilling through the tibia (lower leg bone) in order to thread the tendon graft through the hole to the right spot for attachment. This type of tunnel is called a transtibial tunnel.
The tunnel drilling technique used until recently often placed the graft in a vertical (up and down) position. As a result of the slightly off-anatomic position, the knee could end up unstable even though the graft was intact. This could be the situation you find yourself in.
It’s also possible that there are other soft tissue or bony structures that are damaged in that knee and still require repair or reconstruction. Sometimes these additional areas of injury go undetected until after the primary reconstructive surgery fails.
A second look is warranted and usually requires additional imaging studies (MRIs, CT scans). You may want to consider seeing a different surgeon for a second (independent) opinion. Without a complete history and physical examination any further thoughts would be purely speculation on our part and unfounded by any real evidence.