The 5 micron discs typically have loose tolerances from our supplier (meaning they tend to fall out like you are experiencing, because they are slightly smaller than the slot for them to fit in within the sample vessel cover). What you need to do is take a “center punch” tool. This is similar to a punch used by woodworkers to punch nails into the wood past the surface. A center punch can also simply be a piece of stainless steel that is the shape of a pencil like a traditional nail. You want to slightly “deform” the edge of the 5-micron filter part by making the edge stick out further so when you press the disc back into the filter assembly it stays in position.
Place the disc on the work bench and use the center punch or similar tool to punch the edge of the disc with a hammer on 4 sides (90 degrees apart), deforming the edge slightly making the edge wider and thus giving the filter assembly holder something to grab onto when you press the filter back into the assembly.