If the stutch you just worked has pulled some of the slack from the next stitch you are about to work it can appear that this stitch is very tight and hard to get into. You can try not to distort each stitch too much as you work into it, enough pressure to get your working needle through but without pulling on the stitch too much.
As @kushami said it can feel tight trying to get into a K2tog, when I find mine a little tight in the stitch I insert my right needle purlwise into that second stitch and give it a gentle pull with the needle to draw up a little slack from the stitch either side to make it easier to get into. I do not work the stitch in this direction of course, this is only to loosen the stitch a little, then i take out the needle and work as usual. On some occasions as I you’ve it little purlwise pull I use my left thumb to hold this stitch pushed up against the keft needle, this keeps the new bit of slack to the front and sitting slightly proud of the needle, leaving a small gap when I can put my right needle for the k2tog.
Sometimes I notice my overall tension alters if my hands are cold or a little damp from having washed my hands, it can cause friction between my hand and yarn on my tensioning and prevent the yarn gliding properly. Maybe this happens to you too.