Digital signature not visible

For 4 months the visible signature worked perfect.

But today when I make a visible signature it is not visible.

It seems to be kind of periodic:

I tried one PDF document, and the visible signature was not visible

Then I tried with another document, and the first time the signature was visible.

When I tried more times on the original second document, the signature was every time invisible.

I use Windows 7, 32 bit, and Adobe Acrobat XI 11.0.10.

Regards Finn