Manual focus through the viewfinder is difficult, especially at f1.4, where your
DOF will be very thin. It's possible that you are coming close but not hitting the perfect focus point.
I would recommend trying this:
-use a tripod or set the camera stable on a table
-focus as good as you can on a consistent object (maybe a wall square with the camera, or maybe one small object jutting up into the frame without anything around it) through the viewfinder
-without moving or bumping the camera, switch to liveview
-if you press "ok" in liveview, the camera will zoom in (by default) on the center of the image. you can use the thumb wheel to zoom in closer, all the way to 10x or 16x. you can move the magnified area around with the direction buttons. this (with focus peaking) is the best manual focus aid the camera has.
-you'll be able to tell about how far off you were using the viewfinder by correcting focus zoomed in like this. pay attention to which way you have to turn the focus ring.
-if you repeat that several times, I would expect you will find that sometimes you're focused a bit in front and sometimes you're focused a bit behind where you meant to be, and that it's kind of random each time but your'e always close.
Basically the DSLR viewfinder without any focusing aid isn't extremely accurate. You can really think you've nailed it, but it's more of a good guess. I've
added a split prism focusing screen to my K1, and it's still really a challenge. Especially where I'm looking for super critical focus, and especially wide open, and especially with a subject relatively close to the camera for a given lens. A
DOF calculator may help understand better what you're up against. Manual focus isn't easy!
Now, if you're having some other problem, like;
-what's in focus in liveview isn't in focus in the file once you've snapped a shot
-if it's happening stopped way down or with distant subjects
-if you're finding you are very consistently off in the same direction by the same amount
well then I'm not sure and that could be more specific to your lens or camera.
BTW I don't have that lens, although I have several wide aperture manual focus lenses and a K1. I wish I had that lens