Combining Collections

Combining Collections

I have several "collections" which are almost complete. The 7th box sometimes says COMPLETE, even when I am missing one of the first 5 items.

When I have all 5 items, why doesn't the final box change to COMPLETE? Instead, it gives me another box (6th) of items that I can BUY, to combine a collection.

Example: I have the 5 items of Colorful Set, yet the collection has not been COMPLETED.

I don't understand the buying process. I've tried to buy items, (one or two,) it takes my crystals but it doesn't complete the combining process.

Here is an example of my current Colorful Set collection.
The 6th box that says COMBINE, offers the following:
Winding Key. It says 12/4 cost 4 crystals
Spring: 9/4 cost 5 crystals
Gear: 1/4 cost 6 crystals
Mechanism: 0/4 costs 12 crystals.

The last box, Colored Pencil Set is NOT activated, and I cannot figure out HOW to get these last boxes to activate.

(sometimes they have done it, on their own. Perhaps I obtained an unknown piece which completed the set??)

Questions is this:
1. How do I use the COMBINE box?
Is there a downside to using the COMBINE box?

2. What do the numbers mean? (scenario re-pasted below)
Winding Key. It says 12/4 cost 4 crystals

3. If Winding Key says 12/4, does this mean I have 4, but need another 8 keys? Cost is 4 crystals, does this mean I have to use 32 crystals to buy this Winding Key?

There is no way I will have enough crystals to buy ALL of the items in the COMBINE box. Especially if I have to buy several of them.

In additon to WInding Key, are these other 3 items are needed to COMBINE?
Here is an example of my current Colorful Set collection.
The 6th box that says COMBINE, offers the following:

Spring: 9/4 cost 5 crystals
Gear: 1/4 cost 6 crystals
Mechanism: 0/4 costs 12 crystals

I hope the question makes sense.
Appreciate any help with this! I love the game, but feel like I'm doing things wrong and want to learn.

THANK YOU.