Short answer. The court will issue an order of execution against the non-paying party, either on its own initiative or on motion, without prejudice to holding them liable for contempt. A third person who advanced the support in the meantime can also seek a writ of execution to be reimbursed.
What the law says
If the adverse party fails to comply with an order granting support pendente lite, the court shall, motu proprio or upon motion, issue an order of execution against him, without prejudice to his liability for contempt.
Rule 61, Section 5 — Enforcement of order. Read the full provision →
The court can act even without a motion
Enforcement of a support pendente lite order does not have to wait for the applicant to formally ask for it. If the ordered party fails to comply, the court shall, motu proprio or upon motion, issue an order of execution — meaning the court itself may initiate enforcement, on top of an applicant's own motion for the same relief.
Execution and contempt are separate, stackable consequences
The order of execution is issued without prejudice to his liability for contempt. That means pursuing execution to collect the unpaid support does not use up or replace the possibility of also holding the non-paying party in contempt of court for disobeying the order — the two remedies exist side by side, not as alternatives to each other, and pursuing one does not require giving up the other.
Someone who stepped in and covered the support can also collect
The rule also protects a third person who furnished that support to the applicant when the ordered party refused or failed to provide it. After due notice and hearing in the same case, that third person may obtain their own writ of execution to enforce a right of reimbursement against the person who was actually ordered to give the support — recognizing that someone else, such as a relative or friend, may have had to fill the gap in the meantime rather than let the applicant go without support.
Why this matters for someone relying on the order
An order for support pendente lite is meant to meet real, ongoing needs while a case is pending — a party who is not receiving it is not without recourse, and does not have to simply wait for the underlying case to be resolved. Knowing that execution can follow, and that a family member or friend who steps in to cover the shortfall can later seek reimbursement, matters for anyone caught in the gap between an order and actual payment. Neither remedy requires starting an entirely separate case; both proceed within the same proceeding that produced the order.