Greetings Ricardo,
Do you have Material Requirements Planning activated?
Reserving a stock component (lager; item category L) in a PM Order does not trigger a stock purchase automatically; it's up to the purchasers to provide these materials to stock. If you feel you are overstocked on spare parts, perhaps it is your MRP settings or the stock replenishment process that could be reviewed?
All a reservation does is to create a stock requirement for a material to you PM Order. Are you saying that those requirements are often not processed (i.e. you tend to reserve a material, but not issue that from stock to the Order?) In that case, you may want to use an unplanned issue process (wherein you may, for example, issue stock material that was not prior reserved when confirming an Order)
Also: Do you use non-stock materials (non-lager; item category N) for your PM Orders as well?