A L5+ enhancement is equivalent of a L6 one, and a L5++ is equivalent of a level 7. Therefore, assuming you were L3, you could use up to L5+/6 enhancements, but not L5++/6+/7 ones. This means that you can't upgrade that enhancement, so the game tries to replace it instead.
Buying enhancements to update others is generally not worth it, it's just more convenient to replace them every 5 levels (Starting from L12, with DO's, before that it's a bit different).
That is assuming you're not using IO's, which complicate things somewhat.
And if the enhancement you have is showing as green, then it is higher level than your current level, so what Max is saying about it not being able to combine due to level is quite possible. If the enc is the same level as you, it shows white, lower level is yellow and it goes red when it's no longer giving you a boost.
Disclaimer: The above may be humerous, or at least may be an attempt at humour. Try reading it that way.
Posts are OOC unless noted to be IC, or in an IC thread.
if you have a 5+ anything and try to "drop" another on top to combine it, sometimes it comes up thinking you are "replacing" rather than "combining"
if you aren't sure, click the enhancement you want to "plus up" and if it appears fully colored in the enhancement combination screen then it should be availiable to be enhanced further. If its slightly greyed out, you need to level up before you can enhance again.
In-game and now on Twitter @Tsumiju Zero "The Nightmare of Dra'Gon"
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BTW you can't improve a 5++ with anything else then a 7 or higher. If you used a 7 to enhance it with, the new enhancement would become a 7+.
Uuurrrrgggghhhh.... numbers.... all these numbers.... maths-maths-scary-maths-arrrrrrggghhh!!!!!
*takes a moment to lie down until head stops spinning and breathing returns to normal.*
I have now reached a conclusion; I am too stupid to play this game. 
Thanks to all of you guys for taking the time to explain the logistics behind it to me - I appreciate your efforts greatly, particularly since you could not have known that you were in fact speaking to a total numbers chump. 
I think I shall just do what you advised me to do, TsumijiZero - I didn't even know you could do that, so thanks for the heads up!
Geez, I hope there aren't any mobs that attack with fractions or complex equations in this game... I really WOULD be doomed...! 
The short way is to offer the Enhancement up to the slot if the numbers are green slot it if not don't 
Not very scientific but that's how I do it.
I leave the mega builds and maths to others I just play 
and enjoy.
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Yep, me again.
It's possible I'm being a plank by asking this question, but hey, that's never stopped me before so here goes...
This seems to occur when I buy both enhancements from the same Quartermaster AND when I buy from two different ones. Even more mysteriously, this problem doesn't seem to occur once I've got high enough to buy level 10+ enhancements. Is this a bug, or is there some logical reason I'm unaware of as to why this should happen?
As I understand it, when you buy an enhancement and slot it, you can combine it with two more enhancements of the same type and level (or lower, within allowable range) before you get a message saying that you're trying to replace it with a lower-level enhancement. So, for example, if at level 5 I put a level 5 Health enhancement in Twilight Grasp, I can put in a further 2 level 5 Health Enhancements (or level 4 or 3 instead) so that the corresponding slot reads "5++", before I get the aforementioned message (and have to wait until I can get an enhancement of a higher level than 5?)
Well, this is an experience I have been having, and it's been confusing me:
I go to a Quartermaster, and buy a level 5 enhancement and slot it. I go off, do a mission or two and then go back to the Quartermaster. I decide to treat myself to another level 5 enhancement to combine with the existing one I bought earlier - I haven't found any as drops to add to it, so it's the only way to upgrade it right now. It's still showing as green in my menu, so it's not too low-level or anything, and, as I said, I've only got one existing enhancement slotted there so far. I buy another one, try to slot it and...
I get a message saying I'm trying to replace it with one of a lower level! But it's the same as the one I've already slotted, also bought previously from the Quartermaster.