Reading a File while it is being written by another process

I'm trying to read binary data from a buffer file which is continuously written to by a different process (that I cannot modify). I'm using the following code in order to open the file:

fileH = CreateFileA((LPCSTR)filename,
                    GENERIC_READ,
                    FILE_SHARE_READ | FILE_SHARE_WRITE,
                    NULL,
                    OPEN_EXISTING,
                    FILE_ATTRIBUTE_NORMAL, NULL);

And it opens correctly with no error. However, when I read data from the file, it seems to block the other process from writing to the file since I loss data.

The buffer is circular, meaning that the file size is fixed, and new data is constantly written over older data in the buffer.

EDIT: Sometimes the most trivial solution works...

I've contacted the software company and told them about the bug, and within a day they posted a new version with a fix. Sorry this cannot work for everybody.