I suspect you expect subscripted symbols to just work. To make them robust enough for executable code, you need to work on them.
The attached notebook has a brief introduction to making subscripted variables that are reliable in executable code.
Also, using capital letters as variable names is risky, since many of them have built-in meanings.
p.s. Your post arrived the day the Student Forum was officially closed. Please check out the new Wolfram Community at http://community.wolfram.com/.
Attachment: SubscriptedVariables101.nb, URL: ,