Whether you "need" a skimmer is debatable. Some claim that if you don't have one that your beneficial bacteria will grow accordingly and stabilize your tank, but a few members here tried to go skimmerless, with unsatifactory results.
Food and fish waste converts to protein 3 stages before it reaches becoming ammonia. Removing it at the protein stage seems the most sanitary to me.
If your skimmer doesn't work, get a different one. I have no experience with in sump skimmers so if you have a sump, you can ask for a simple reliable skimmer, but if it's to hang on the back, lemme recommend the Aqua C Remora Pro. Love that thing! Works awesome!!