Yes this is a constant part of my life and I have come up with several work arounds. Little background, I do product development, sorta a one stop shop for businesses looking for a hired gun to work either alongside their engineering staff or as a skunkworks project(joshgoreworks.com). Not all products are consumer goods, some are airplanes, or structural engineering projects, or power tools, or medical devices, and some are pure as seen on tv(sometimes it pays the bills). The below is what I do when the creative juices feel dry, although I do believe that is merely a perception, not reality, the mind can always be woken up. I know that if I really have a lapse in productivity I have the nuclear option... which is to remind myself just how capable humans are by exposing myself to serious discomfort. Be it camping in the rain, spending a day doing manual labor unrelated to my goals, or simply fasting... yes fasting.
But here are my go to things.. I do this stuff daily.
1. I took to actually sketching my sketches in real life in a notebook far prior to opening any software. The engagement I get by drawing the steps required to produce the geometry keeps me engaged until the design has enough novelty to capture my attention. This is unrelated to the actual industrial design I might do for the customer, this comes later when those designs have to go into cad and I need to start understanding the future associativity required to make the cad work for me. I learned this technique early in my career when doing class A Surface modeling for bottles and other packaging. Many times the industrial design would not meet the volume requirements of the package and required adjusting Height, Width and Depth... If I sketched out in advance how I would control that associativity in the event the package needed 100 more ml of volume, the history and associativity wouldn't break when making the change. This process really would get me 'into' the cad.
2. I force myself into the shop a few nights a week or a few days a week even if it isn't required. If I didn't have my hands in prototyping and development I would not be on the computer at this point in my career and would have moved to management.
3. I keep a number of running lame duck projects. These are projects for which I have no goal, I have no axe to grind, and nothing to prove just rather a pure curiosity about what could be. These are things that I may never make, nobody paid me to design, and nobody is interested in licensing(not that I know). Often times I will be 100% stuck on a basic design solution, jump onto my personal projects, see how I solved something there(when nothing was on the line but my own interest), then apply it elsewhere. If I really can't get motivated I will open up a lame duck project and add a few features, ones I was dreading, and then all of a sudden I feel drawn back towards my 'real work'.
4. Change of scenery. If I can't get settled into the cad I will grab the laptop and just work from somewhere else for awhile.
5. Straight up physical exertion. Many of my 'lame duck' projects involve physical activity, meaning building something I test involving physical input, be it a bicycle, or a part that needs to be bolted onto a car and then tested, or even a rc plane or anything that changes up my usual routine.
I'm sure I will think of more, but yah I totally understand and it can be super annoying to know what comes next but can't seem to get it designed, or if you can you are not happy with the results nor motivated to change it. I have 3 designs on the wall which need to be in cad tonight with rev changes on the drawings so it can go into the shop tomorrow. If I didn't get the 3d printer rocking those parts right now to review in an hour, and promise the customer and vendor they would have drawings I probably would be trying to find something else to do like make an elaborate dinner, which when its done would have taken as much time as just finishing the drawings and ordering food.
That was a long winded way of saying I get it.