@Fotis_flex
The preview mode only displays a subset of data. Save and publish and check again in the analysis.
Others things you can further validate :
1/ If the join keys exists in both, possibly check there are no whitespaces in one of the columns.
2/ You can even create 2 separate datasets. Bring both datasets into the analysis . Create a visual from each dataset and bring the join key column ( use table format ) , add a filter based on the join key (filter settings : all applicable visuals ) and check if they are visible in the analysis.
Kind regards,
Koushik