Polti Vaporetto 1500 safety cap won’t screw off.

\015 Polti Vaporetto 1500 not used for a few weeks and just went to put safety cap on ( just to make sure that I had put in the inner white bit in correctly ) and then couldnt unscrew the cap off so that I could add the water. Tried for ages and eventually it unscrewed it self. Then added the water and did a second check of unscrewing the cap and again unable to unscrew it. ( Nothing to do with the steam pressure as not put the unit on yet) Any advice. I am wondering if the thread of the thread of the screw is worn. Machine is about 10 years old but I only use it about once every 2-3 months. The only thing I noticed is that it felt a little tough to put screw on. ( I have had contractors in my property who accidently flooded my property and when I returned from work they had packed all my belongings on top of each other in a corner and looking back I am thinking that stuff was piled on top of the Polti ( which was in a large bag) ) Any ideas or tips to try to unscrew it.\015 – Polti Vacuums