What is your PS2's model (The many PS2 models in this world are all slightly different on the inside)?
When you mean "White cap", did you mean the white magnetic disc that presses down on any CD/DVD that gets inserted into the CD/DVD drive?
If so, shouldn't it be part of the drive's top cover? So you took that out of the cover, or did you just take off the entire cover along with the white cap?
If you had really taken the cap out of the cover, then I can't help you.
Maybe someone else who's more experienced with the PS2's hardware can help you.
But if you simply can't position the cover right, shifting it around until the magnet "snaps" to the drive's rotor should help in getting the cover positioned correctly.
BTW: Welcome to PSX-scene!
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