Training with different datasets for the same better VAE model yields poor results

The VAE model I used here https://github.com/keras-team/keras-io/blob/master/examples/generative/vae.py.
It can produce very well results for the minist and fashion minist dataset.

But when I use my datasets, the results are pretty terrible. So I sincerely hope you can help me to provide me some guidance.

The problems are as followings:

1. My datasets are composed of many circles in the image, as shown:

The shape of the datasets is [238, 28, 28, 1], and the range of pixel value is between 0 and 1 (floats32).

2. But the result is terrible.
   The loss value
   
   The genetive images: