Normally communities put in an extra payment during the year as and when something major comes up that needs sorting and there's not enough money in the kitty to pay for it.
In our community, if this needs to be done, an EGM is called, the cost is agreed and the amount each owner has to pay too.
In the early days of our community we had a similar problem to yourselves. We had many empty properties and some where the owners never paid. So we used to have to compensate for this in calculating the amount each owner has to pay, so that we could get the work done.
As these costs are agreed at an EGM, it means that every owner has to pay it. For those properties that are unsold or the owners don't pay, that extra fee becomes a debt associated with the property. In terms of the unsold properties, when somebody comes to buy one they (should) be informed by their solicitor what the outstanding debts are that need paying. The buyer normally then asks these debts to be either removed from the price or be paid directly to the community.
In fact, when properties get bought, you'll find that the associated debts normally get cleared quite quickly afterwards.
Hope that makes some sense.