Somehow I understand what you are saying, although 'circle cast' is usually used to refer to the circle spey or snap-c. Sounds to me like the momentum of your line coming upriver is overcoming your d-loops ability to form 180 degrees to target.
Internet only offers blind analysis and these symptoms could have several potential causes - however...
- The application of power in the stroke differs significantly between the switch and the single spey. I would recommend thinking about the two casts as distinctly different casts. Although general appearances would make one think they are very similar, the separation of the two is key to mastering the direction change IMHO.
- Begin with the shallow dip technique until you get a feel of the mechanics. A gradual dip introduces a slow spot in the line against which you can pull in a different direction. Do not underpower or overpower the line tension. An abrupt dip introduces turbulence into the line and anchor so the keyword is "smooth". Slowly reduce the dip to a minimum, virtually eliminate it - this may take months or years.
Note that a switch cast requires no dip, although the rod may come downward to level out a high lift. The application of power with / without a dip is different, and this is important to turning corners.
- Try video analysis. It will tell you volumes about what is happening. Set up a tripod past your d-loop's range and film yourself making the angle change cast. Watch what is happening. I limit myself to 30 minutes of taping between reviews. Watch the film, then make a plan for the next session. You will have copious feedback about what is happening and can devise an improvement plan for the next 30 minute session. You will be surprised at how quickly you advance with video feedback especially if you have slow-mo and freezeframe playback.
Getting a solid single spey down is like getting to Carnegie Hall - practice, practice, practice
Online analysis can sometimes help but spend some time with an instructor when/if you can, there is no substitute for first-hand analysis and suggestions that pertain one-to-one with what you are actually doing.
good luck!