Hello.
I'm really good at HFSS software simulations.
My offer is: I will make the HFSS model of your antenna. The slot dimensions and microstrip feeder line parameters will be set as variables - I will optimize them. Perhaps, your patch antenna will work better. I'm not sure yet since I didn't see it. Maybe it is an electrically small device, or ground plane too small. If your device doesn't have size limits - return coefficient, sure, will become better.
Later, after we figure out the dimensions, we can do a changes to your CST model.
You choose:
I've access to CST, but need time, since I've used CST only a few times. It's just matter of time. Two weeks will be enough.
or
If we have tight time limits, you may change the antenna dimensions to the new calculated values by yourself.
Please let me know about your thoughts.
Thank you.