If it’s correctly reassembled but still won’t stay attached, the most likely cause is wear in the retaining tabs/nubs rather than user error. In the reported case, the small nubs that hold the wide panel in place had worn down enough that the panel would no longer stay secured. If the spring and tabs are seated properly and it still pops off, there usually isn’t a simple adjustment to fix it. For a rental unit, the right solution is to return or exchange the flash rather than force the panel or attempt a repair.