Need help for edexcel 9-1 history

Title pretty much says it. In particular, I think its the exam technique I need help with.
WARNING LONG READ

The other day, I was handed back my history essay(Explain developments of government policy towards Plains Indians 1936-1961). It was an 8 mark question and I only got 6. Now here's the thing. I was so sure that I tried to do everything right. I have bought myself a textbook to help me answer these questions but to no avail. I put down all the information in the textbook to answer the question but still end up with a measly mark. I covered the two pointers and added one of my own. the teacher said that I should include information that covers the whole time period in the question. Is this necessary? Because I'm very clueless. I just wrote about 1851 because it is in the time frame. Probably a dumb mistake?
Also any other tips on how to write a good history answer. I just suck at exam technique in general. I know a lot of people in my class who are good at history even more so on TSR so there must be something I am doing wrong but my ignorance prevents me from stepping forward.

So to sum up: What are the things you need fora good history answer(to get full marks)?

And to clarify: Is history a regurgitation subject? Can you just open the floodgates in your brain and let out the knowledge? That might be the problem.

Six is a good mark and the teacher was generous. Have a look at mark schemes from the exam site if you wnat to know what the marks are for.

Imo your mistake is you didnt read the question and you should have covered the whole period or most of it, not just one year. The measly mark as you call it is down to yur own lack of observation..

How many points should one write for an 8 mark question as a rule of thumb?