This one is hard to say without seeing your work. Here's my shot in the dark thoughts.
These two stitches are sharing thread between them. Break the steps down.
- Slip one
- Pass slipped stitch over.
If you normally K1 one and then adjust the tension by pulling on your lead yarn, maybe you are not doing that between the K1 (step 2) and the PSSO (step 3) stitch? If you are not doing this, there could be extra yarn in step 2, that you can pull up into step 3 to make things loosey-goosey.
Or when you do step 3, your could be pulling it really high over the stitch and squeezing the just knitted stitch down real tight and making the psso stitch too loose.
If none of that makes sense, then write back where you have looseness developing.
Please let me know if this was helpful or if you are still stuck.