Awareness is your most natural or primary form of existence, and it is always in existence effortlessly throughout your experience.
When you are deep in work that is familiar to you, you get immersed or in a "flow" state because your mind agrees with the work for one reason or another, and therefore it is at ease being identified with the work. This clears the way for you to fully take the seat of your natural state of awareness and experience its effortlessness. Any relevant thought flows freely out of this pure awareness. Your mind and awareness are at one with the work.
When the work is new, surprising, or complicated, your mind takes the lead and begins to identify with the challenge, trouble, or effort it is experiencing. This identity is cemented through conscious or subconscious thoughts like "I have to figure this out!" or "I can't figure this out!". This creates an energy of effortful strain that blocks the feeling of flow that you experience when you are fully in the seat of your awareness.
In order to fully take the seat of your effortless awareness while doing complicated work, you need to see that the work has nothing to do with your true, purest identity, which is the awareness itself. The attachment to "having to figure it out" must be let go so that you can relax, be open, and in acceptance of any possible outcome whether or not it may be deemed favorable.
While this may seem simple conceptually, it's not necessarily easy because of the habitual nature of the mind to create or attach to an identity that is not awareness. It often does this when it is taken off guard by high-contrast / reactive situations.
Meditation is a great way to confirm through experience that you are not your mind, thoughts or effort, and to build your mind's trust that it can detach from any other false identity. This will allow you to fully take the seat of your most pure identity, awareness.
Occasionally try looking directly at your awareness - become aware of your awareness - and try being aware of it in the context of your thoughts and identification tendencies.
This direct experience or "seeing" will gradually (and sometimes quickly) change the mind's habitual nature so that it won't react because of its identification position relative to the work. This trust within the mind will eventually draw you fully into the seat of awareness without a meditative exercise. This will enable you to undertake the thought process with great clarity and without a strenuous feeling, no matter the complexity of the work or situation.