Every single player is able to reach 5 stars in every hotel. Objects paid with gems won’t affect this progress. You should reach the proper number of stars to unlock new hotels.

If you find all maxed out in Imperator Palace but you don’t get the fifth star, please check the casino objects manually, it sometimes could give error and it could have a missing upgrade on your premium poker table.

 
You can easily check which room or hotel service needs to be maxed out by tapping on the stats button (right side icons of the playing screen). You will find a progress bar for all your rooms and also a tab with all your hotel services and their progress percentage. You just need to tap the “Go To” button to find the pending upgrades. 
 
The game also includes a yellow progress indicator in every dialog window of rooms and hotel services. ALL of them must be at 100%, as well as all the staff hired.   
 
Please, double-check ALL your services and rooms objects, such as parking features (security cameras in Blue Crystal Resort), restaurant decorations (especially at Stanley Resort, because you need to scroll down), hire all the employees available, etc. Once you fully upgrade the hotel, you will get five stars.