I had the exact same issue facing hard problems in my work. I noticed that my main issue was keeping focus. My solution was to introduce on distraction free 90 mins slot of work in the morning (my most productive time) to tackle the hardest problems. No e-mails, no phone and no de-railing to other topics. The results so far have been great. The inspiration came from Deep Work [1] by Cal Newport. The book describes the philosophy behind it.