Mechanical plugs to seal the damaged condenser tubes

Hi;
We're having problems with the pop-a-plug type mechanical plugs we use to seal the damaged condenser tubes. It's a combined cycle power plant with one or two daily start-ups. Does anyone have experience with this problem? Are there any plugs or solutions on the market that guarantee these plugs won't fall out? The plugs we're using, recommended by the condenser manufacturer, are the ones that keep falling out. They fall out periodically, coinciding with the filling of the condenser water tanks during plant operations, causing contamination of the cycle with seawater.
Regards,