Computer B and C can ping A, but A can’t ping B or C

I’m a bit new to networking and I have the following Problem:

I have three computers:

A (windows10)
B (windows10)
C (ubuntu server, raspberry)

If I run the ping command:

A to B -> Request timed out.

A to C -> Destination host unreachable.

B to A -> works.

B to C -> works.

C to A -> works.

C to B -> works.

All computers are connected to the same modem via ethernet cables. The weird part is, that the connection from A to B and C worked yesterday but not anymore.. If you need more information please let me know.

Thanks for helping 🙂