"But i tried to map that value from the flat file to any other doc/string list " → How is your loop step look like FF loop to other doclist loop? It could be your loop and map steps underneath that might be not mapped correctly (source list to target list)
I think its a mapping assignment problem and can you make sure the source iterations Flatfile/recordWithNoID has the values and so when you explicit map to Doc/recordWithNoID will map the iteration accordingly? Please debug or prefer go with a loop step with InArray/OutArray to explicit map the source to target field.
Else do straight high mapping from Flatfile/recordWithNoID link to Doc/recordWithNoID assuming the fields beneath both the doclists have same keys (Name,Class,Roll etc…)