Oh yeah, very common. I've been at this for 2 full years now and I've lost "motivation" many times. Eating new foods, trying new recipes and trying new classes at the gym (or new activities at home) has definitely helped.
Honestly, my
gung-ho motivation ended around month 5 of my journey, when I took my first major vacation...whew! I gained 8 lbs in 2 weeks, but I just picked up where I'd left off when I got home. I kept beating myself up because I'd always gain weight on vacations, business trips and during holidays and then I realized...that happens to a lot of people, even the "normal weight" people.
When I'm really in that "I don't care anymore" mood, which strikes on a normal basis; I will try new things but I also sometimes stop trying to lose weight and instead focus on maintaining my weight. It's not easy when you know you still have more to lose, but it helped me learn a lot about myself. I realized that it wasn't very difficult to maintain - I wasn't always following my eating plan (I do WW online), but as long as I continued working out 3-4 days per week and I didn't have super crazy binges every day, I would maintain. Some weeks, I'd go up a pound or 2, but at least it taught me what I could and couldn't do.
Anyway, the key, of course, is not to give up totally. Even when I concentrated on maintenance, I still didn't go back to my "old" ways of eating and I never stopped working out. Some weeks I was so mentally/emotionally frazzled that I would only concentrate on sticking to my WW plan and my workout plan, but not weighing myself - I did that so I'd make my goal more about living healthfully, rather than trying to lose weight.
I dunno. There's really no sure-fire way, but I'd definitely try and switch things up a bit. Looking at my old "fat" photos or going into a department store and trying new clothes on (in much smaller sizes) was also helpful for a quick boost!