IQ in healthy people is near impossible to improve. But why IQ is not improvable is unknown: many other cognitive abilities are improvable (like working memory), so why not IQ?
Arthur Jensen noted the failure of interventions in the 1960s, and the failure remains complete now, half a century later: if you are a bright healthy young man or woman gifted with an IQ in the 130s, there is nothing you can do to increase your underlying intelligence 20 points. New methods like dual n-back or nootropics are trumpeted in the media, and years later are discovered to increase motivation & not IQ, or to have been overstated, or work only in damaged or deficient subpopulations, or to be statistical artifacts, or to be tantamount to training on IQ tests themselves, or to simply be fraudulent. The only question worth asking is which of these explanations is the real explanation this time.