1. the reason why/that The reason why/that I have failed to improve my English is the absence of good learning materials. "reason why" is common generally suitable for any speaking or writing context. I recommend learning and using this one first. "reason that" is more formal and usually seen more in writing. 2. the cause of - formal The cause of my failure to improve my English is the absence of good learning materials. This is too formal for everyday conversation and writing. 3. the reason for my failure to .... This is just like 2 but perhaps more common and slightly less formal. However, "reason why" is better in conversation. These examples relate to objective situations. Reason can also be used to indicate why a person chose to do something - the reason in their mind. "cause" cannot be used naturally for this.

Michael explained it better than me!

a 'cause and effect' is a direct result of something from something else; What was the cause of the delay? A tree had fallen down on the road. We're talking about the reaction to something, maybe without intention. reason is more normal (an explanation - why??) so, your sentence should be 'I couldn't improve … because there weren't many resources/as there weren't many resources', etc Or, 'The reason I couldn't improve.. was that there weren't..' If someone asks you 'why?', that's your reason.. true or not! 'cause' is not appropriate because it didn't REALLY stop you from improving!

There are many free online resources additionally children’s books can help a lot like Dr. Seuss.