I have downloaded the Belgium AIP on my pc and tablet (--> all plates are Downloaded)
Now i just see 8 aerodromes. EBAW, EBBR, EBCI, EBGB, EBKT, EBLG, ELLX and ABOS.
Not the other aerodromes, e.g. Spa, EBSP.
When i click on the Spa aerodrome on my pc, it says: There are no plates available for this specified airfield.

The Belgium AIP is free, so why can't i see the other plates?

Unfortunately that is not the same international AIP they license to third parties through the Eurocontrol PAMS system. In that AIP, all the "smaller" airfield details are grouped together into one page in the non-aerodrome-specific part of the AIP which you can find in the SkyDemon document browser.