The reason it is not getting aggregated because there is a time dimension in the module. Every Time Period is treated as a separate block and you can't use SUM across them. I might be wrong in saying so but that is my understanding on it.
Edit: Here is a snip from one of the finest articles ever written on Anaplan by @Griffink Full article Here
Now @Ankitjain if we take a look at your requirement to sum it in the same module. Does that data belong to the time dimension or the period line item, if it belongs to period line item you don't need time dimension, replace it with any list and then you can easily use SUM on period line item but if it belongs to Time dimension then period line becomes irrelevant and you don't need to use SUM at all. You can simply aggregate it using TIMESUM or SELECT on All periods etc etc.