Jet

Hello! Fellow Pogo player here 🙂

I think I understand what your problem is. You can't get past Denver and on to other cities, correct?

Each destination requires you to get a certain number of stars before it opens up a new city... check your stars.

If this is not it, there is arrow you can click on that will move you to the next page, which will reveal the next city.

Lastly click on the blue button on the bottom left that looks like a world, this will take you to the home page of Jet Set Solitaire, this will reveal all the World Tours Available.

I have attached a picture to help as well. 

If none of this works stop spending gems, there is definitely something wrong. I doubled check and played a few games myself and it works fine, normally bugs are systemwide and not personal, so there may be an issue with your computer graphics, especially if it is over 5-7 years old.