If foam re-forms after initial collapse, it could indicate that the defoaming agent used may not be sufficient or appropriate for your system. Here’s what you can do in such cases:

1. Increase Dosage: The simplest course of action to start with is to increase the dosage of the defoaming agent. However, this should be done in increments to avoid overcompensation which can lead to other issues such as wetting problems or residues.

2. Evaluate Defoamer Type: Different defoaming agents have different working principles and chemical compositions. If increasing the dosage doesn’t solve the issue, it might be necessary to consider switching to a different type of defoaming agent.

3. Alter Application Method: Sometimes, how and where the defoamer is added can significantly impact its effectiveness. Consider altering the application method, such as using a more dispersed injection across the entire system.

4. Evaluate System Parameters: Parameters like temperature, pH, agitation speed, and chemical compositions can affect both the formation of foam and the effectiveness of a defoaming agent. You may need to control and adjust these factors in your process.

5. Consult With the Manufacturer or a Specialist: If all else fails, it might be time to reach out to your defoaming agent manufacturer or a foam control specialist. They can provide additional guidance or recommend alternative products.