For those who weren't aware and using the Rubicon kits, they aren't actually polystyrene, they are ABS and so the standard polystyrene cements won't work well. You have to use a proper ABS glue like 'Plastic Magic'.
I spent ages wondering why bits of my Rubicon kits would actually fall off after being glued.
OP was specific that his question was about Perry Plastics. That aside I am not sure why this is happening to you. The Rubicon kits I have built went together perfectly well with the Humbrol polystyrene cement from the tube. Maybe it depends on how old your glue is? Perhaps newer glues have a different formula to suit both polystyrene and ABS?
From Humbrol/Airfix Website
Humbrol Poly Cement
Usage - A solvent-based cement suitable for plastic model kits only.
Substrate - Common plastics used to manufacture model kits including polystyrene and ABS. The product works by melting the plastic on application and ‘welding’ two glued pieces of plastic together to form a strong bond.
Application - Directly onto plastic model kits. A cocktail stick can be useful to apply sparingly to small areas or fine details.
Drying Time - Surface dry in 10-20 minutes. Fully dry in 1 hour.
How to Clean - Can be removed if not fully dry by re-dissolving with Humbrol Liquid Poly or Precision Poly. Product is permanent once dry.