GroupIntoShards not sending bytes further when dealing with huge amount of data

[Eddy G]

As seen in this image https://imgur.com/a/wrZET97, I'm trying to deal with late 
data (intentionally stopped my consumer so data has been accumulating for 
several days now). Now, with the following Window... I'm using Beam 2.27 and 
Flink 1.12.

                            
Window.into(FixedWindows.of(Duration.standardMinutes(10)))

And several parsing stages after, once it's time to write within the ParquetIO 
stage...

                            FileIO
                                .<String, MyClass>writeDynamic()
                                .by(...)
                                .via(...)
                                .to(...)
                                .withNaming(...)
                                .withDestinationCoder(StringUtf8Coder.of())
                                .withNumShards(options.getNumShards())

it won't send bytes across all stages so no data is being written, still it 
accumulates in the first stage seen in the image and won't go further than that.

Any reason why this may be happening? Wrong windowing strategy?