I have found I have a problem recently. I have reached the stage in the skills I practice (guitar, and learning languages) where I have gotten to the point where what I am able to do in these skills are no longer challenging, but I cannot progress.
Using languages (specifically German) as an example. I have progressed to the point where I could live in the country and survive, but not to the point where I can express what I would like to. I can go shopping, eat, and pretty much survive, but I could not hold a conversation with a friend for long.
Does anyone else have the problem of not progressing in a skill, or had the problem and have overcome it? How would I start progressing again?
No, but I've done a lot of reading on my own.
Maybe you need to surround yourself with harder adversaries? Like you said, you can shop in German. Next, why not join a German chat group in the internet and try to keep up with native speakers or better yet, join a German group in person or go to Germany and try to keep up orally. I have met many people professionally who have come to my English-speaking country from their native land with the belief that they excelled at English only to humbled as they attempted to communicate with native speakers (as opposed to their fellow second-language speakers they were superior to back home). To grow we all need a challenge.
I was never able to propel my self directly upwards when climbing with just my arms, I started climbing with a rucksack that weighed 45 pounds every evening at the gym, after about a month I could throw myself up to a new hard point with just my arms and not fall, I could also climb tougher routes in the wild with fewer cams.
Your solutions is to find a native german to speak with.
Where are you? Do you live near a university or college? If so I am sure they have a German club you may be able to join or at least visit. What about your church? Is there anyone there that came from Germany? Look for a German heritage club in your area. Visit Germany for a year or so.
As to the guitar, there is no not-challenging. Can you make it speak? Can you play like Jimmy? Can you play the steel guitar? You're not setting your goals high enough is what I think. You reach a point and say "look what I've accomplished"; while you have accomplished a lot you don't see the summit above you because you're looking down at where you've been and not up where you should be going.