Is your workflow set to trigger on Item Update? I doubt on this.
There are checkbox in the Edit Workflow page.
you should enable "Start workflow automatically when an item is changed"
Please let me know in case more help is required.

EDITED ANSWER as per OP's Comment.
Devang / Vince - Yes after select "then update item in current list" but "Start workflow automatically when an item is changed" now its updating. But its treating as a new value. For example (First Deliver Qty 200-10=190 then 190-15=175) so here if I'm editing the Qty from 15 to 20 so it will show the result (190-20=170) but result which I'm getting is (175-20=155) which is wrong. Any suggestion please. –
Add One more column in list- Previous Balance.
While you are editing the record second time,
Set Previous Balance= Old value of Qty, in your case 15
Then Qty= New Qty, in your case 20.
Keep in Mind- When you are adding new record- set Previous Balance=0 or empty.
So as per your comment
Previous Balance= 15
and then Qty= 20.
Now change your Workflow logic
The step where you are deducting the quantity
Add if condition
If Current Item:Previous Balance is not empty
Set Deliver Qty to 200+Previous Balance-Qty
Else
Set Deliver Qty to 200-Qty
Further EDIT 2:
then Calculate Variable: Balance Qty minus Current Item:Delevery Qty (Output to vriable:calc)
Change this to
If Current Item:Previous Balance is not empty
Calculate Variable: Balance Qty plus Current Item:Previous Balance (Output to Variable: calc )
Calculate Variable: calc minus Current Item:Delevery Qty (Output to Variable: calc )
Else
Calculate Variable: Balance Qty minus Current Item:Delevery Qty (Output to Variable: calc )