Loose roof lining in poptop bed = Bedouin tent!

I have a two year old Hymercar with a popup roof containing a double bed. The lining on the "ceiling" of the pop top has unglued itself so that it sags impressively in the centre, giving occupants the impression of being in a tent, not to mention having very limited headroom at the narrow end of the pop top. The original glue is still on the underside of the roof and does occasionally do its job for a bit after the pop top has been fixed in the down position for a while, but it never lasts and the "tent" always returns.

I watched a Youtube video of a method of repairing a sagging car headlining where the maestro, armed with a tube of glue with a plunger at one end and a hypodermic needle he had cunningly attached to the business end, injected the glue through the headlining fabric onto the underside of the car roof and then manually pushed the fabric upwards until the glue took hold. He was probably someone with a nursing qualification who had sidelined into car upholstery and for that reason appeared considerably better qualified and suited the this particular task than an average bodger like me could ever hope to be!

Has anyone else (who has not yet featured on Youtube) experienced a similar problem and identified an effective way of repairing this sort of problem?