I spent hours of labor making sure my shop area (190 sq ft) was well sealed as to not let any hot air in or cold air out. I am cooling with a 25,000 BTU unit. The problem is I have a service counter with a sliding window that I installed so I could air condition the room. When this window is closed the room doesn't cool and in fact the temperature actually slowly climbs. When I open the window the temperature drops fast and the room cools efficiently to a certain extent as now I am letting the warmer air in. Air circulation issue? How do I correct this so I can keep the window closed and still have the room cool?

25,000 BTU is to big for a 190 square ft area,, get you a humidistat and see what the humidity reading is . Guessing its very high,, not only will you feel hot and sticky, you are looking for a mold and mildew problem

Are you using a through the wall unit or a split system? Does the unit start to cool then stop? (when window closed) What does the unit do... cycle often, quit running? My first thought is that the unit is oversized and freezing up, blocking the air flow. Can you describe the layout of the room?