Mine is also the same, but i figure out it's the senior. It has nothing to do with how strong the hatch is slammed.
Just a little trick I found out to get it to work properly. If the hatch didn't lock completely. ( you will hear two beeps), don't re-slam it. Just give it a hard push on the Subaru logo, it will beep one more time and confirmed fully lock. Kind of stupid, but works.