What may be different reasons that I score less(30.00/100.00) on some problems and get wrong answer for some test cases but AC for the earlier test runs(that fetch me those 30 pts)?
Facing the same problem here too!
Each subtask has some range of input variables. Lets say n for one subtask is 10 and for second is 10^15. If you’re allocating an array of size n, then you’ll easily pass first subtask but not second. SImilarly, there can be cases when you get TLE or you might be missing some corner test cases of some subtask which is giving you WA. Or may be your types of variables (int, long int etc) are not correctly specified in your code according to the given input. These cases occur very often!!
Considering that types of variables have been specified properly by me, there is no problem of tle and array size has been appropriately specified, what could be other reasons for WA in 2nd subtask? In my case, ALL the test cases are reporting WA for 2nd subtask, not just 1 or 2, so I am not sure if corner cases could be a cause.
Usually, there is some particular type of case in all test files. I am saying this only because of what I have learnt from past contests. Sometimes use of inappropriate datatype outputs some garbage value, or may be mistake in your logic(for particular type of test case). I cannot tell that during the contest 
Only thing I can say, try all different types of inputs as per the constraints and chekc the answer manually and match with your code’s output.