You can compare the traffic snapshots of recording and replay, using LR VuGen, or some 3rd party capture tools like Fiddler.
If the issue is caused by client side requests during replay (for example, some dynamic values are not correlatd), the server response should contains some error messages, sometimes in HTTP protocol level (error code), sometimes in HTTP body. If the failed server response info has no difference to the successful responses, you need to turn to server side app logs, or ask server side developer to do some troubleshooting.