I have to agree with this statement by renaissance_man here, a bit more info on your process would help narrow down some answers for you.
Is the bust you're working on all made from the same material and which areas don't seem to be curing? The latter part of this question is leaning towards the idea that maybe you have made up several batches of silicone and the potential that quite possibly the catalyst hasn't been fully mixed in with some of those batches.
If you could describe in some detail what you did, the order that you did it and what you were using, we may be able to throw some ideas your way. There could even be a contamination issue... Some silicones can be inhibited from curing just because latex gloves were worn when mixing and applying this material.
I doubt very much popping your mould into a sunny area will help, the process is a chemical cure (exothermic) and not via evaporation, like you'd find with latex.
Hopefully the hive mind here can get you some help.
It could be that you brushed it on too thick. If your work area was too cold. If you mixed the silicone incorrectly. If you used a catalyst it might not of been shaken enough before being added... As the others mentioned, more info on the material used would help.
I’d guess it was the room temperature; was very cold. Also when mixing I did not add the correct amount twice out of about 7 layers I brushed on, I was in a hurry and did not think it would matter much... I thought maybe to brush one more layer on top of it all to harden? However I’m not sure if underneath it’s cured or not
"I did not add the correct amount twice out of about 7 layers "
I was going to say not mixing enough but i would bet this is your problem. You have to mix every batch the proper ratio and mix every batch well. It cures slow so there is no reason to skimp on either step.
I would just step back and wait a few days, if you used too little activator then it could take days or longer to cure.
If you can take something away from this it's you have learnt a lesson on ratios. I've made plenty of mistakes over the years (including silicone ratios) and it certainly makes you a better prop maker in doing so.
As zorg says, leave it a few days - even a week - and see how it's looking. If everything turns out golden, I'd also suggest that you give it a few more passes of silicone (mixed correctly) and smooth out all those peaks and troughs as that'll be a bugger for registration when you come to lay up your hard shell.
Rebound 25 is usually super reliable. If the layers on either side of the poorly mixed layers were well mixed, cross your fingers and give it some time. I apply super thin layers, over and over, on top of each other.
If the inside layer, the one touching the mold is poorly mixed, start over. That first layer is critical for preserving detail!
For me, the outside is usually more drippy but smooth looking. I also cut little pieces of yellow foam and embed those in the middle layers to help make registration blocks and cover it with Freeform Air. When making a hard outer shell for the Rebound, actually have holes where the registration pieces bulge out, it helps create a vacuum to hold the Rebound to the hard outer shell.
If you cannot wait and don't mind the wasted Rebound, pull it off and start over. Otherwise, I'd give it a few days and see if it cures.
The original is blue, 3d printed, did a lot of smoothing with XTC and sanding. Ghost in the Shell Geisha face
I looked at your pic again. I am hoping the lighting is why the color seems to vary. If the Rebound is so poorly mixed that it has color changes, you may want to get a toothpick or something and mixing up the still wet, uncured Rebound.
I would always premix before pouring. Even if you poured some 20 minutes ago. Scrape the sides. Then mix the poured stuff for atleast 60 seconds. I count it out.