If you are an officer or a board member you should be seeking the advice of an attorney in your area for answers to your questions and to represent the HOA. An anonymous forum such as this is not the place to seek advice when you have an obligation to act in the best interests of others as that obligation includes obtaining advice from a qualified professional.
Who is responsible for the payment of dues? The owner of the unit. What can the HOA do when those dues are not paid? The HOA can file a lien against the unit and/or sue the owner. The lien will not be satisfied if the property is sold at foreclosure. In that event, the only recourse the HOA has is to file suit against the owner, obtain a judgment, and collect it.
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If condo is worth less than the balance of mortgage(s) the foreclosure won't produce any money to pay HOA even if you now file a lien against the property. If condo is worth more than what is owed, then HOA could be paid out of foreclosure proceeds, but HOA would need to record a lien against the property before foreclosure sale happens.
The other option is to sue the homeowner and get a judgment against them. Whether the homeowner has anything: wages/money you could collect against is another question. Depending on how much is owed, potential recovery, it might not be worth pursuing the debtor. If you contact a creditor's attorney they can give you more details about the process and costs involved.