As innovative and fantastic as it is, the PS2 system is fraught with disc reading issues due to problems with the laser over time. Though backwardly compatible, many times the PS2 will have problems reading PSX games functionally. And unlike with the standard PS2, the laser in the slim version cannot be simply adjusted by hand.
Pick up your PS2 slim and flip it over so that the laser which reads the disc is pointed downwards. Gently shake the unit from side to side. Sometimes the tracking will become disjointed over time. This may coax the laser back into its original position and allow it to read discs without colliding with them.
Turn the PS2 system on its side with the disc drive at the top of the unit. Unlike all systems prior to it, the PS2 is capable of functioning both flat on the ground, as well as erected vertically. This will often aid in the reading of discs otherwise unreadable.
Replace laser or buy a new unit. Unfortunately, with the PS2 slim it is no longer possible to hand adjust the laser. Once the first two steps no longer help the problem, it is time to either buy a new unit or have the laser replaced. Servicing your PS2 slim unit can be done online by accessing: "https://service1.us.PlayStation.com" and selecting the PS2 slim icon from the list of PlayStation products.