Ability to teach what you already know to people who are paying attention is certainly a trainable skill.

Ability to learn specific cases within a given field is almost certainly a trainable skill; with the common elements tucked away in long-term memory, there's more short-term memory available for whatever makes the specific case unusual.

Ability and willingness to pay attention is probably a trainable skill, with other limiting factors.

Fully-generalized ability to learn is... difficult to disentangle from general intelligence. The big gains so far in general intelligence seem to be either genetic, or a matter of removing penalties like malnutrition and lead poisoning. If we had a proven, generalized way to learn how to learn faster, iterating it would pretty much be the Singularity.