I did really well with stitch in the ditch on my first few quilts. Then I did one that puckered everywhere. It doesn't sound like you're doing anything wrong. What kind of batting are you using? Poly batting is quite slippery and can be a real pucker pain! I find I'm doing better now with all cotton batting (warm & natural or warm & white). The fabric and batting tend to stay in place better. I still pin a lot.
FMQ can help reduce puckers because you are kind of easing any fullness in as you go, instead of having it build up to the end of a line of stitching.
You'll get it, just keep trying until you find what works for you.