It sounds like you may have lime or hard water deposits around the screw threads on the cap from the water used to create the steam in your steamer.
I use white distilled vinegar, that I -

• first warm in the microwave
• then slowly pour over the sides of the cap so that the vinegar can get "under" and around the cap.
  • I also stop and try to tighten and loosen the cap trying to work the vinegar into the threads.
  • Depending on the severity of the lime build up, depends on whether the cap comes off the first time, you may need to repeat pouring the warm vinegar over the cap or soaking the cap in the vinegar overnight (with a vinegar soaked rag) - I hope it helps??