Dear Raksha,
Unfortunately, the Volumetric sources can’t be used with the multiple spot sources. To have multiple cylinders as your sources you need to use a source routine. (On slide 34 you see pencil beams with 0.25 cm radius.)
I would suggest to take a look at your colleague’s Jyoti Garg’s (@jyoti) problem, since she is using a similar source routine:
Cheers,
David