Tube with reversed faces... how this happened?

On a face I have drawn circle and then a tube from it. The end needs to be formed like on the image. First I tried TGI-3D’s Cross Sections tool, but the faces disappeared. This already happened to me but I don’t see why. I revered changes on the cylinder and tried different method. I have selected the ending circle (bottom on the image), selected Move tool, pressed ctrl and copied the circle. Using shift on blue axis, I have move the circle up and I have change the diameter. This way I copied the second circle and did similar. This way I have formed the bottom of the tube. But some faces disappeared (and as I remember, there were already this reverted faces when I have placed the copied circles on the cylinder’s surface. So, using soft pen I have added the faces which were missing. And now I need to understand what I did wrong that this reverted faces has been created and how to correct them.

I add two images.

With hidden geometry ON - so you see the facets’ edges - you have two ‘fix’ options…
If it’s part of a solid object, then select a correctly oriented face and right-click > context-menu > Orient … to make all of the connected faces face the same way.
If it’s not part of a solid, then you need to select the incorrectly oriented faces, then right-click > context-menu > Reverse… to make those faces face the same way.

The reverse faces are probably not a result of your errors, but rather a result of the way SketchUp is struggling to interpret face orientation in complex forms.

Either way, the ‘fix’ is relatively straightforward…

They are simply fixed as Tig has explained and don’t require some other external tool that none of your posts about it have so far proved it to work the way you think.

I recalled there were no faces after I moved the circles onto the tube. Then I selected the edges and generated faces. So this is the problem of the faces generator. Now I have experienced same thing on different object. Anyway it is mystery why the faces disappear sometimes.

No, this is pilot error.
It is a simple problem easily fixed by learning the basics of the software.

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Are those faces very small?

Yeah, the object is about 3.5 mm in diameter.

→ Work at a bigger scale to prevent faces from disappearing…

So what should I do? To create model of size 2x2x4m and to make a tube (or pin) of size e.g. 500 cm on it? Then to create component from it and resize it to model of e.g. 20x20x40 mm?

Very much like that. Work 1000x full size using metres as if they are millimetres. Then scale the result down by 0.001.

You can do as @john_mcclenahan says - work in meters [without any units showing] as if the units are mm.
Because you are working x1000 the tiny geometry problem is avoided.
When you are done scale down again…

Alternatively, start the object at the desired size in mm.
Making tiny edges and faces can get flaky as it approaches SketchUp’s 1/1000" tolerance, and even fail altogether !
Select the geometry and make a component of it.
Use Move+Ctrl to copy the component instance to one side.
Select the copy and Scale it on the diagonal so it is done evenly, x10 [or even x100].
Double-click on the copy to edit it.
Do your potentially problematical operations.
They should succeed at the larger size.
Exit the edit.
Select and delete the giant copy.
Zoom back in to the original and the changes will still be there.
You cannot create tiny geometry, but it can exist !

Very useful. Thank you.