When my mum asked me, why I "was getting" bad marks at school last year.
direct speech "Why were you getting bad marks last year" so reported speech should be "why had you been getting bad marks last year."
I would like to know why no backshifting for" was getting", is it a direct speech ?or it is not necessary to backshift and in this case why(because it is obvious that "were getting" is before the asking)
In short, the rule of always backshift is wrong. It's a general guideline that's okay as a simplistic way of looking at reported speech. But there are contexts where it's optional or even unwanted.
First, let's start with a more obvious example:
Direct: "Why are you getting bad marks this year?"
Reported 1: My mother asked me why I was getting bad marks last year.
Reported 2 : My mother had asked me why I had been getting bad marks that year.
In the direct version, it's in the present tense. In the reported versions, it's in the past tense. I'm assuming that the reported speech is actually coming a year later.
I've added Reported 2 because this is the tense you would use if you were recounting a story. (I will expand on this later.)
But, based on timing, some of this backshifting will not occur.
If you are reporting something that was said minutes ago:
Reported 1: My mother (just) asked me why I am getting bad marks this year.
Here, there is no Reported 2 at all because it's not far enough in the past to be recounted as a story.
If you are reporting something that happened a month ago—but still this year—and it's still happening (if it's still happening and remains factual, backshifting is optional):
Reported 2 is a way of expressing the same thing when you are recounting it as a story. It's likely you'd use it in a broader context.
For instance:
I remember when I turned my grades around last year. My mother had asked me why I had been getting bad marks that year. It made me think, and I realized I needed to work harder.
But if the direct speech were actually in the past to start with, as in your question (let's assume it's still only one year in the past):
Direct: "Why were you getting bad marks last year?"
Reported 1: My mother asked me why I was getting bad marks last year.
Reported 1b: My mother asked me why I had been getting bad marks last year.
Reported 2 : My mother had asked me why I had been getting bad marks last year.
You don't necessarily need to backshift beyond what you did before because the past tenses in the reported speech could already be appropriate for the content. Note, however, the addition of Reported 1b. The second verb is backshifted further. That's likely the version you would use if you were simply volunteering the information to somebody else. The first version is what you would likely use if somebody asks you what your mother said to you—or if you are asked to recount a sequence of events. ("And then what happened?") But both would be understood.
When it comes to backshifting, you have to consider the context. Timing plays a large role in how you report something—as does the original tense of what was said and whether or not what was said continues to be true or is no longer the case.
