There is something stuck inside the lance/pistol

This is easily solved.😊
When the o-ring holder has loosend from the bayonet and is stuck inside the lance, it would look something like this:

 
The o-ring holder is rather easy to pull out, but if it is quite stuck, one can loosen it by using:
screwdriver
knitting needle
pair of pliers
crochet needle
etc.
The last one is quite handy, as one gets something to pull with at the same time. 
 
Here is how the bayonet looks without the o-ring holder, check to see if the o-ring is still inside the bayonet or needs replacing:

 
Use the AVA First Aid Kit, to replace both o-ring and holder. If you need the bayonet/ nozzle before you are able to get a hold on the AVA First Aid Kit, you can move an o-ring and holder from another attachment to the one with a missing holder. 

https://avastore.co.uk/product/first-aid-kit-for-ava/ 

 
We hope this explanation was helpful.