Is a microswitch ON when slightly pressed?
Figure 1. A snap-action microswitch. Source: Wikipedia. The snap spring is shown at (1).

Most microswitches use a toggle or snap action. The actuator has to go beyond a certain point at which the contact arm "snaps" over to the alternate position. Similarly, on the way back a similar snap takes place. The hysteresis - the difference between switch on and switch off points - prevents the switch from chattering when on the transition.


  So my questions are - is a microswitch in the ON state when pressed in only slightly?


It could be either way. Figure 1 shows a changeover type so by using the common (left) and centre we get a normally open switch.


  ... and can a microswitch lose sensitivity over time?


Yes. Parts wear out but it was working before disassembly and doesn't now so ...


  I've tried adjusting the copper microswitch part but this hasn't worked.


Again, looking at Figure 1 you need to be careful where you bend the actuator. In the snap switch pictured you would need to bend the arm somewhere in the region above (1). If you bend it between the pivot and the black button you could lose operation completely.