It is common to see great progress when you are learning a new skill in the beginning and later see your returns on effort diminish at some point. It happens in all walks of life such as athletics, academics and people just being "stuck in a rut" in their careers. Mixing things up a bit and trying a different approach can be a remedy for this problem.
A few examples of this are when a bodybuilder gets stuck at building a muscle group and isn't seeing much growth they will introduce new exercises to shock their mucles back into growth. When learning academics looking at a topic differently can initiate new thought processes to assimilate more information. People stuck in a career may take on different responsibilities to keep things interesting.
The same is true of music. Lets say you are interested primarily in heavy metal. A lot of the songs you work on will have the same techniques then you come across one that is much different. The common approach to overcome this is what I call the "brute force" method. This is when you practice something so repetitively that you finally get it. Unfortunately this approach is time consuming and really boring.
A better way is to get to the root of what is happening in the song through studying the music background it comes from. Take "Dee" by Randy Rhoads (Ozzy Ozbourne guitarist in the early 80's). Dee is a piece with its roots in classical guitar. Even if you have mastered all of Randy's other tunes the techniques you took on in those will be of little help to learning "Dee". The common approach of most players is to take it on by "brute force" which in this case would be ridiculously time consuming. Moreover you would be just learing the notes, not the concept. The only thing you will be getting good at is "Dee" and not be able to apply that knowledge to many other classical pieces.
So instead try mixing it up a bit. Get a classical guitar technique book, in this case "Classic Guitar Technique" by Aaron Shearer would be a good one, and study the musical style that it is written in. This will in the long run help you master "Dee" more quickly and give you a greater understanding of what is going on. Not only that but you can apply this broader knowledge to other songs that bridge classical and rock music. A couple of examples would be "Black Star" by Yngwie Malmsteen and "Spanish Caravan" by the Doors.
You can do this with other influences too. If you wanted to learn jazz influenced "Black Magic Woman" by Santana or "Moondance" by Van Morrison you would study jazz guitar techniques and apply the to those songs. That way you would take away some new techniques that you can apply to any song with those techniques.
You can see how mixing things up stylistically will help you get out of a rut when you are stuck and open new doors to your playing. Who knows you may find that you really enjoy a style that you are exploring and study it with the some excitement that you would your usual one. Overall it will definitely benefit from it.
So next time you are stuck try mixing in something new and see how it works out.