How to tighten a Dollfie Dreams head?

Hello everyone I joined this forum because I have that one question that bugs me for almost a year now XD Last year I got my first Dollfie Dream (Miku Hasune...original no reboot version) and I noticed fast that her head is quite loose. I thought this is standard because I only had her but some time later I got the DD Luka as well and her head is quite firm which is good. SO from there on I knew that maybe Mikus head is too loose. Now I mostly collect Barbie and Bratz dolls and I tried everything I knew from how to tighten Barbie/Bratz heads on my Miku but nothing worked. And no one I asked could tell me how to tighten Mikus head or even tell me for sure if it´s possible. 
I hope someone here can help me out. (with pictures would be perfect because I´m stupid and need pictures to understand 😄)

Loose in what way? Does her head come easily off the neck peg, or does it just flop around a bit like a bobblehead?

If it's the first problem, there might be a crack in the neck hole on her head, which you can repair. See here for an example of that:

If it's her head won't stay in a position you put it in, there is probably something wrong with the spring mechanism on the neck joint. Miku's head may seem a little worse in this respect than other DDs because her twintail wig is so heavy.

If there is a problem with the neck joint, while that's not really made to be taken apart, you can do so. Look at pictures about 2/3 down the page here:

https://ameblo.jp/kotarovision/entry-12356873601.html

Or to replace it, you need the upper torso frame:

https://ec.volks.co.jp/item/4518992398457.html

Volks is soon discontinuing these parts so how long they remain available isn't yet clear.

Thank you for your reply.

I´m still not sure what the problem is. I guess it´s the part of the upper torso frame where the smal metal piece is. Not sure how to explain it but I don´t feel comfortable to screw any screws on dollfie dream dolls. I feel like if I would mess up I couldn´t fix it anymore. I could somehow attach a rubber band between the ball part and the part that is pushed up with a spring and it made the head a little more stiff but it´s still loose ^^°
Not sure if I could explain it well ^^°

On 10/26/2019 at 6:24 PM, ShinobisDestiny said:

Thank you for your reply.

I´m still not sure what the problem is. I guess it´s the part of the upper torso frame where the smal metal piece is. Not sure how to explain it but I don´t feel comfortable to screw any screws on dollfie dream dolls. I feel like if I would mess up I couldn´t fix it anymore. I could somehow attach a rubber band between the ball part and the part that is pushed up with a spring and it made the head a little more stiff but it´s still loose ^^°
Not sure if I could explain it well ^^°

You can also get a neck replacement part from CoolCats.

Dollfie Dream do require some user maintenance, so at some point you are going to have to deal with taking them apart and fixing things. As cfx suggested you can get a replacement part from Volks, and that is going to be your easiest replacement. Basically you only have to do same steps you do to swap a bust when replacing the upper frame.

Thank you all for the help. I tried some things. I didn´t bought any new frame pieces but I tried my luck with rubber bands and saran wrap. It´s not the most good looking way but it already helps a lot