The problem that many students face is the mechanics of how we create sounds. You will often hear people say that the sounds of RP are aspirated from the front of the mouth.
But the question we have to ask is how is this achieved? You see, most phonetics books draw attention to the positioning of the articulators (the mouth, the tongue, the lips and the teeth, etc) to explain how to produce the sounds. For some students, this may be enough if they get it intuitively. However, what may seem intuitive to one person can be incredibly challenging for another.
For example, if I’m teaching a student from a General American English background, most of the focus will be on avoiding rhotic sounds, lightening the /l/ sounds, avoiding slacking the jaw and making some slight adjustments to the vowel sounds. After all, British English and American English are both English. Aspiration and syllable stress and rhythmic patterns between the two accents already share much in common. In this case, making slight adjustments to the positioning of the tongue and the shape of the mouth can easily result in a natural British English pronunciation. This is why I am confident that I can train an American English speaker to adopt a native-like British accent in very few lessons.
However, the subtle mechanism that we use to aspirate sounds varies among the different languages and this is influenced by the region of the vocal tract from where the sounds are aspirated. Some languages can be more guttural, some languages can be more nasal, some languages can sound higher pitched and other languages can be lower pitched. Take the cockney accent for example. The difference between RP and cockney isn’t just simply a difference in pronunciation. The sounds of cockney are more gutteral so it’s not just about glottal stops.
Accents go beyond just the positioning of the articulators and this is why so many non-native English speakers struggle to attain native-like pronunciation when speaking English.
We also have to consider the rate of airflow which can affect syllable stress. These aspects go beyond the simple placement of the tongue and when students try to apply the same mechanisms of their native language to that of English, they run into problems. The British tongue does not move in rapid succession in the same manner as a Spanish, Italian or Japanese speaker. In English, the articulators make deeper contact for mere fractions of a second creating more resistance between the articulators. Once the student can develop this intuition and get a feel for the interrelation between, the lungs, the articulators and aspiration, they will make good progress.