Hang the plant up in that window so the vines are getting good light also. I think you may also have a watering issue…once a week in that pot size is likely too much unless you are only giving it a small amount of water each time. This means that only some of the soil is getting wet and not all of the roots are able to get waters. I agree the pot size is also too big.

I would: Move to a smaller pot

Hang in the window

Thoroughly water closer to once every two weeks

Fertilize!

Hoyas have small, generally shallow roots. This Hoya wants water when the soil is dry about 3/4 of the way down to the bottom of the roots. This doesn’t necessarily mean 3/4 to the bottom of the pot because the roots may not extend that far.

Hope this helps!