Are you shooting with the cutout hood in place? Try it without the hood. The original fibre was red on my sights. I changed mine to green, as the human eye is more sensitive to green.
There's a tradeoff between small and sortof bright, which is still good out to 200yds, versus big, bright and fuzzy which is barely usefull at 100yds. Personally, I like the small fibre.
The Marlin cutout hoods block too much light, so I remove the hood when I'm in the field. I use the hood to protect the sight in storage and transit.