Likely a bad cable especially if the device you use to test gets faster speeds plugged in with a short cable directly.
Hard to say all depends on the details of how you have this installed. Generally it is the connection at the ends that has issues with in wall cabling. You should be able to reterminate them. The wire itself is seldom the cause unless you purchased fake cable. The wire must be pure copper (no cca) and have wire size 22-24 (no thin or flat cables). These restrictions are much more important when you are running longer distance though walls where it is hard to replace.
Cables are really strange when they are bad. They can work on some devices but not others. Us home users can not afford the equipment to...