open raman solid sample add-on spectrum problem

[Fabulut]

Hello all,
I have been working on the open raman project with the 40mW laser.
Luc has been helpin a lot in the process, I want to thank him again here.
I first started with the liquid samples and able get a spectrum.
Then I aligned for the solid quvette and get the raman spectrum of paracetamol. I added the picture.

paracetamol.jpg (26.7 KiB) Viewed 1018 times

I also tried the sodium carbonate and get a nice spectrum. Also added the picture

carbonate.jpg (26.48 KiB) Viewed 1018 times

But not every sample I have, give a raman spectrum. I tried with the grapphite and got nothing.

graphite1.jpg (35.3 KiB) Viewed 1018 times

I also tried lithium metal oxides and not able to get a spectrum.
In my experience the light coloured samples gives raman spectrum but not dark ones.
Have anoyone ever experienced similar issue, if so can you give me any pointers to overcome this problem.
Thanks all for your help