Filament not sticking to print bed  

Clean your bed

If your first-layer calibration is still fine the best thing you can do is cleaning your bed.

If you touched the print area barehanded you transferred grease to the sheet that you can't clean with IPA alone.

You need to clean the sheet with soapy water. There are many detailed instructions here in the forum.

RE: Clean your bed

Thank you for the suggestions. I have done it several times.- The layer 1 calibration prints fine, but when it starts printing the model, the filament (most of the time) ends up becoming a ball around the nozzle.

Other ideas

Make sure your nozzle is clean. You can use a soft wire brush to clean it, or try to print one of the nozzle cleaners you can download from Prusaprinters.org. Mostly an issue with PETG but good practice got PLA as well. You can also use a silicon shoe to protect your hotend from accumulating gunk. 

Besides that, you said your first layer is fine but then you shouldn't see problems with your actual models. I hope youre using this method https://forum.prusa3d.com/forum/original-prusa-i3-mk3s-mk3-assembly-and-first-prints-troubleshooting/life-adjust-z-my-way/, not the built in one, to calibrate your first layer. If you send some first layer pictures we can take a look. 

Which print bed have you got?

Please show us a picture of your first layer calibration.

Cheerio,

Filament not sticking to print bed

as you can see...it starts printing, at one point, the whole things starts moving. 

I have cleaned the bed surface

Clean the nozzle

adjust the layer 1 calibration

changed the filament

did it improved? yes

problem in video

Images

You might have two problems.  You definitely have under extrusion and you might not have you Z offset correct.

Filament not sticking to print bed

What do you recommend?

reply

If filament doesn't stick to bed, then there can only be two reasons for that: either you first layer calibration is wrong (you need to lower Z-offset) or print sheet is not clean.
As cwbullet mentioned (above), there can also be under-extrusion problem. But you can be sure about that only after you made first layer calibration correctly. That is, don't try to find solution for under-extrusion as long your print doesn't stick properly to bed!
There are many reasons that can lead to under-extrusion, especially if you changed some "sensitive" settings in slicer. On mechanical side, there can be issue with hotend (cleaning and replacing PTFE tube might be necessary), or there's problem with extruder (clean filament grip gears of residue, extruder calibration might be necessary).

RE: Filament not sticking to print bed

Your first layer calibration would be more useful but from your pictures there are two issues to address first:

Clean the print-bed again.  There are smears visible in your pictures.  Use dishwashing soap (Dawn/Fairy) and lots of HOT water.  Dry with fresh paper towels and handle by the edges only.

And revisit your first layer calibration, some of those threads look too round, not squished enough.

At the end of the calibration you are aiming to print a single layer *sheet*. It should be possible to peel it off in one and fold it without the threads seperating. If it breaks into lines it is too high, not squished enough. If it is a single sheet but with wavy lines or drag marks on it you are too low.

For most people it pays to use 7x7 mesh bed levelling.

Many beginners have problems with the built-in first layer calibration routine. There are alternatives, see this page by a regular poster here: http://projects.ttlexceeded.com/3dprinting_mk3_live_z_calibration.html

and look at this thread: https://forum.prusa3d.com/forum/original-prusa-i3-mk3s-mk3-assembly-and-first-prints-troubleshooting/life-adjust-z-my-way/

Cheerio,