I'm going to keep this short, sweet and straight to the point.
There is a lot of issues with the heat-seeking system on SAES when you are dogfighting. To sum down the issue, watch the following video:
As you can see on the above video, there's a clear lock on the opposing hydra, however, the missiles aren't heat-seeking whatsoever. This has been an issue for many years, and it was sadly never given attention; people just ignored it.
There's a lot of servers that has reworked the system (don't ask me how), which improves the system by a large margin, to limit the number of bugs (such as the one above) to occur when you're dogfighting.
It is extremely frustrating, as there is nothing you can do except for praying your no-scopes hit the target.