Analysis of water pressure leakage in high-pressure plate heat exchangers

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Recently, I was testing a 5-flow plate heat exchanger with a capacity of 30 bar for a DN300 system by applying static pressure to the equipment alone; this unit consists of around 500 plates. But after 4 attempts at pressure testing, it still didn’t pass. The details are as follows: 1. When the test pressure was applied, water dripped from one spot; upon disassembly, it was found that the glue applied under the rubber strip was a bit thick. 2. After removing the rubber strip and replacing it, when the pressure reached 20, one of the rubber strips suddenly popped out of its position, causing severe water leakage. 3. The rubber strip was checked and found to be fine; it was reinstalled with an additional 10 mm of compression. At a pressure of 30, there was no leakage, and after holding the pressure for two minutes, yet another rubber strip popped out of its position. 4. The rubber strip was checked again and found to be okay; the position of the plate was changed along with the rubber strip, but at a pressure of 28, another rubber strip popped out of its position. The equipment was inspected, and the plates and rubber strips seemed to be fine; the fixing plates and movable plates were 110 mm thick with no signs of deformation. What could be the reasons for such sudden heavy leakage? Additionally, when performing strength calculations, the use of a screw that is one size smaller resulted in insufficient strength. I wonder if it has anything to do with this? Please give me some advice! Why did the rubber strip suddenly pop out?