As monarchs usually leased their land to szlachta (or merchants, or Jews) for short-term benefits, they didn't have direct control over cash flow and this often resulted in corruption and delayed payment of military wages.
Military commanders (hetmans) often had to pay the wages of wojsko kwarciane and hired mercenaries from their own purse.
Sometimes disgruntled unpaid units formed confederations (see konfederacja) in order to lawfully collect their wages by force.
It was a relatively small standing army when compared with other contemporary countries, especially considering the Commonwealth's huge area.
Commonwealth armies would almost always be stretched thin to defend its territory from its aggressive neighbours of (Sweden, Russia, the Ottoman Empire and its vassals).