Siemens s7-300 downloading error

Hello All,
I am facing problem while downloading the S7-300 ladder to CPU. I tried browsing the Forum if someone had this kind of error, but couldn't come across any.
The ladder has been compiled tested on simulator first & runs fine, but when I download it to CPU it does not allow me to download any FC's FB's OBs. I checked the MMC card & its present. I have tried Mreset , online deletion of all blocks in the CPU memory & even downloading a truncated ladder which contains just OB1 with 1 N/W. Still the problem persists.
My hardware configuration is CP315-2DP, CP341 in Processor rack, Rack1 -Rack 5 containing I/O cards . All racks connected on DP/Profibus using IM153-1.

Any suggestions welcome.

Thanks,

Make sure that you are not trying to download any marker memory, inputs, outputs or data blocks that exceed this CPU's limitations

What message do you get when trying to download ?

While downloading the FCs & FBs it gives "The Block is too big" . The actual size of each is between 20-30 bytes.
Thanks