There are seals because it is important that the plate is tight on the inside of the basket but they can slip if you try to insert the plates dry.
Try dipping them into water or spraying with sanitiser before inserting. You can also try dusting the plats with grain dust. It is also a good idea to ease them in on an angle and then straighten up to make it easier.
Alternatively, our resident brewer has a helpful trick: Try inserting the basket at the start when heating the water. When the water is around 104°F (40°C) the steam and condensation will help with the installation process.
Start with the bottom perforated plate at about a 300-degree angle and insert into the grain basket so that the lowest edge of the perforated plate is aimed at the inside bottom edge of the grain basket. Once the bottom edge of the perforated plate meets the bottom edge of the grain basket flatten out the assembly gently.
Using the flat edge of your mash paddle tap down the perforated plate assembly by tapping down the edge of the plate seal where it meets the grain basket edge, work around the edge until the assembly is at the bottom of the grain basket. This will ensure that it is inserted correctly and there's no tilting of the plate to let the grain fall through.