Hi Ish,
Please check WBS element hierarchy structure in BW, check it once from top to bottom.
Use to code - RSH1(bw side), select your hier name and double click. check each ndoes how defined.
or go to respective info object at RSA1, select your info object, double click,click on hierarchy tab, click on Maintain hierarchies, on that screen select your hier, its take you to next screen(where you get the same screen by using RSH1). check how hier str look like.
if every things like perfect then, as sugested by suman already, replicate data source into bw, activate data source and further steps also(transformations,dtp and info pack).
Mean while check at RSA3(ecc system), take 5 recrords which your getting error(related nodes) and see them at RSA3.
Thanks