Getting bored with one's main practice I feel often comes from not doing enough of it to experience it as the source of blessings, etc.
I think this is very, very true. Eventually the main practice just "clicks" and becomes more dynamic, much easier to visualize and get into the full-on feeling of being the deity without it feeling like something one is superimposing on oneself, and just more fulfilling. And of course, how much practice is enough for this to happen will be unique to each individual; for some, a lot of practice will be required, and for others somewhat less will do the trick, depending on karma and especially tendrel with their main practice. So perseverance is key.
One thing one of my gurus, ChNN, has said, however, is that it may actually be a good idea in the beginning to do different practices from time to time in order to prevent boredom and burnout, or to avert it if it's arisen, in order to keep one's practice fresh. My experience has been that this has indeed been helpful and kind of led to a boost of inspiration to go back to the main practice with a lot more "momentum" behind it. The possibility to approach it this way will of course depend on not having received instructions from one's lama not to deviate from one's main practice, as for some people powering through may be the better way, especially if their lama can sense they're on the verge of a breakthrough with that main practice. Anyway, just some thoughts. Of course, when I say "main practice" here, I really mean main deity
practice, since our main practice as Nyingmapas is actually guru yoga. (Same may be true for other lineages, but I'm not familiar with them to know if that's the case.)
It's pretty natural for all of us to have to work through this IMHO.