So, you've nailed basic DJing, can match beats, and keep a good set running. You are feeling songs that really flow together and then those that don't. Why? These two songs are the same style and tempo but don't mix well at all, so what else needs to be considered to make the perfect mix?
Depending on your musical theory background, you may or may not have heard about key signatures. A key signature is a group of notes that sound good together and are the "ingredient" notes of a song. Most songs use a specific key signature and only use those notes.
It would make sense then, if two songs each with the same key signature were to mix, the notes wouldn't clash and it would sound harmonically good. This is the essence of harmonic mixing.
Sadly, not every song is the same key signature and so not all mixes can be in the same key. But, there are very similar key signatures. Let's look at C Major and G Major, they are very similar the only difference is the F note which is sharpened in G Major. As you can see they are next to each other in the wheel above. The same applies for the key of F Major, only one note differs from C Major. This is how the Camelot Wheel works.
Every key signature is assigned a number and the greater the number difference, the worse it will sound harmonically.
There are two circles that make up the Camelot Wheel. The inner one represents the minor keys (letter A) and the outer one represents the major keys (letter B). The way this works is the relative major and minor keys e.g 8A and 8B have the exact same notes in them so will sound good together.
If a song is in F Minor (4A), then you could mix it well with another 4A or a 4B where it is harmonically perfect. The next best thing would be mixing it with a song in either 3A or 5A where only one note would clash. Then the next again would be 2A and 6A where there are two clashing notes.
With this wheel and system, you will be able to make your mixes flow smoother and it will be more aurally pleasing to the audience. I recommend you print out a version of the Camelot Wheel so it can help you practice.