What causes this to happen & how can I fix it?

what causes this to happen & how can I fix it?
First tell us what ballcock that you have in the tank. It sounds like the one with the can instead of a float ball. Am I right? Tom

I have the one w/o the ball, like the second diagram.

i have the one w/o the ball, like the second diagram. I'm confused, The one without the float ball is the first diagram. Which ballcock do you have?

Float ball= is the lower one
The Can= the upper one

You are absolutely right, that's what I meant after I hit the submit button, sorry, thank you.

Lulur did you know you have 24 hours to edit or change your post? That's one of the options at the bottom of the post. Tell Tom if the can is down near the bottom of your tank or is it stuck up high ppushing the arm up.

Answer Bobs question first and if the can's OK go on to my solution
So you have a FluidMaster ball cock. This is a common problem with these ballcocks. They have a small washerwith a hole,(see image) in the center that swells shut in time preventing water from entering the tank.
Purchase a FluifMaster Seal#242 from your local big box store. To replace it you will have to shut the water off to the tank and take the tank lid off. Twist the top of the ballcock off counterclockwise, (see image) and pull up. Pry out the seal and replace it. Reassemble, turn the water back on and give it a flush. Better now? Regards, Tom

Thanks for your help, I will let you know tomorrow. You guys are very nice, I am glad I asked. Hope this works.

Best regards,
Lulu