To get out of slump, I may switch to learning basics of another language (for those easy rewards), or I might switch to some other method for a while that keeps me moving forward and/or is otherwise rewarding.
For example, I might stop doing exercises for a while and just watch Spanish TV and be satisfied whenever I completely understand what's being discussed.
Or I might take a step back and think what it is exactly that I'm not able to learn -- and then find new sources that would explain me the same concept but in a different words. Some things one can learn with textbooks, or grammar books, while others may require a good video or personal lesson to begin understanding them.
Yes. And I've also seen this in many students.
As with the other comments above I agree that this is especially true at Intermediate level.
When you are a Beginner there are lots of easy 'rewards'. For example: learn the word 'car', translate it into your language = almost instant success. And then you do this with say another 8 or 9 words in a one hour lesson. And you're happy.
But how many words do you think you are learning at Intermediate level? Consider that you probably need to hear/read/use this word 8 or 9 times to learn it...so you are probably learning 100s or 1000s of words at the same time.
And then there is context. This gets much more difficult at Intermediate level. Suddenly you realise that even a simple word like 'car' has more than one meaning...and the sentence "I took a First Class car to London" means the person travelled by train! ;-)
Therefore success is much harder to achieve, and you have to wait longer for it.
Motivation can be the key. If you are studying for a potentially life-changing reason, maybe to get a new job or move to another country, it can be easy to lose sight of this. Or to think that your dreams are unachievable. Sometimes you need to hear a success story. Here is one:
I know a student who has been learning a language and other studies for years, because they wanted to move to another country and change career. For the last few years I have been teaching them English. They faced many setbacks and negative responses. At times they must have wanted to give up. But they didn't. After taking some risks (moving to another country without a job), and being flexible (changing some of their goals), they met someone who saw their commitment and talent. Today they told me that they have just been offered their "dream job". Perhaps there is an element of luck, but this person deserved it. So don't give up hope! :-)