Some levels are more difficult than others. Make sure that you have all the Upgrades!. Remember that upgrading stations earns you more money (ie: at the Massage Station, performing Hot Rocks at level 1 will earn you $28, level 2 earns $40, and level 3 earns $65).
If you have all the Upgrades and are still unable pass the level you can use Extra-Time Boosters. This helps when you still have customers in the Spa but you did not have enough time to cash them out.
Another big help is hiring Employees. Employees speed up your services so that you can get more customers through and they also earn you 2x the money tips. note: you can still pass levels without employees, its just harder.
You can also check out Advance Tips Section to help you enhance your strategy.