Hi Kay,
The center bulk from the piecing is a common problem. You want to twirl those seams.
Start at about the 33 minute part of the video.
https://www.youtube.com/watch?v=U9BuBROKIrk
A second one for you.
https://www.youtube.com/watch?v=rXSVFY2t8Y8
It does matter how you put the HST through the machine when you put the 4 different ones together to get the twirl. You may have to try it both ways and see which one works for you.