Hotel services have a certain serving capacity depending on how many staff you hire.  

You could see how many people could be served simultaneously in each service dialogue window. Each worker could serve up to 2 people at the same time. 

If a client waits too much, he/she will get mad and won't use any service until the checkout.
The red icon will be different depending on which service you should hire more employees. Investigate what's going on in your hotel!