This could be due to one of a few reasons. Firstly, your pasta water wasn't starchy enough. Make sure you take the water out of the pasta pan at the end of cooking. Secondly, once you add the water and the ricotta mixture to the pasta, increase the heat to medium to help the excess moisture evaporate and the sauce thicken. Finally, it could be due to not properly draining the pasta. Whichever reason it may be, 30 seconds on higher heat should solve the problem.