[Lucifer Matt]

If a nanoparticle formation takes place between a compound   X (reducing & capping agent) : Metal, at 1000 µM : 500 µM then shouldn't that same reaction take place at 100 µM : 50 µM ratio?

I cannot observe any color change at the latter ratio for formation of colloidal Nps. Moreover, my Nps formed at first conc are sticking to the sides of the glass test tube despite giving a rose pink hue.

[Corribus]

Concentration may be too low: reaction is slow.

Glass is notoriously bad for gold sticking. Make sure it is very clean (aqua regia cleaning of all glassware before synthesis of metal nanoparticles is standard procedure). If this doesn't help, try a different type of glass.

[wildfyr]

Nanoparticle formation is not a simple linear process. Ostwald ripening is an important part of the phenomenon that has concentration considerations, for instance.