What makes a puzzle very hard?
At this level, finding the next digit is seldom enough. Work with candidate sets, locked candidates, naked pairs and more advanced patterns. An X-Wing, for example, links two rows with two columns to enable safe eliminations.
Even if a technique does not place a digit immediately, eliminating candidates can unlock the next certain step. Revisit the affected cells after every elimination.
A reliable solving routine
Keep pencil marks accurate. After each elimination, check for singles again: complex techniques often unlock a simple next step.