A pin is only considered fallen when it is knocked down or knocked off the lane surface. In the case you mentioned, where the pin slid but did not fall during your second ball, the pin would be considered left standing, and it would not be a spare. If this had happened during your first ball, the pin needs to be left where it was, and if the pinsetter knocked the pin down afterward, the pin would need to be replaced at its original position.
From the United States Bowling Congress Playing Rules, Rule 6a (Pinfall, Legal Pinfall):
6a. Legal Pinfall
Pins to be credited to a player following a legal
delivery shall include:
- Pins knocked down or off the lane surface by the ball or another pin.
- Pins knocked down or off the lane surface by a pin rebounding from a side partition or rear cushion.
- Pins knocked down or off the lane surface by a pin rebounding from the sweep bar when it is at rest on the pin deck before sweeping dead
wood from the pin deck.
- Pins that lean and touch the kickback or side partition. All such pins are termed dead wood and must be removed before the next
No pins may be conceded, and only pins actually knocked down
or moved entirely off the playing area of the lane surface as a
result of a legal delivery may be counted.
From Rule 7a (Pins, Improperly Set):
No change can be made in the position of any pin(s) left standing
after the bowler’s first delivery, unless:
- The pinsetter moved or misplaced any pin(s), or
- Any standing pin(s) is (are) outside the range of the sweep bar.
Any such pin(s) will be respotted where it (they) originally stood
before the delivery.