I tried both your ideas , but it's not seem to work. I am actually creating the main (master) box first , pic and jauge after. i see the container and the pic but not the jauge until i move or select the "group" , weird..
As you are curious i am using Windev 10 by PcSoft.
Not a common language but really nice in what we are investigating.
So if you got clues why it happens... drop a line

Here the indications
SI cligne="[MACHINE]" ALORS
cligne = fLitLigne(hwnd)
TANTQUE cligne[[1]] <> "[" ET cligne <> "" ET cligne <> EOT
oligneCmd:InitLigne(cligne)
MemAjoute(czmMachines,oligneCmd:GetTrigVal("Machine"),cligne)
//Create main object
objMaster = MyFlowMach:CreationBoite(X,0,200,200)
objMaster>>ZIndex = 0
objActif = objMaster
objMaster>>Text = oligneCmd:GetTrigVal("Machine")
objActif>>Text = oligneCmd:GetTrigVal("Machine")
//Create pic object
oboxpic= MyFlowMach:CreationBoite(X,10,100,90) oboxpic>>LoadPicture(ComplèteRep(fRepExe())+"img\"+"USINE.BMP")
oboxpic>>Text = "MACHINE"+Nom
oboxpic>>ZIndex = 1
//Create jauge object
oboxjauge = MyFlowMach:CreationBoite(100,100,0,0)
oboxjauge>>FillStyle = 2
oboxjauge>>FillColor = 20
oboxjauge>>Text = " % "
oboxjauge>>ZIndex = 2
NewGroup = MyFlowMach:CreationGroupe(objMaster,oboxpic,oboxjauge)
cligne = fLitLigne(hwnd)
objMaster>>PicturePos = 4
objMaster>>TextStyle = 21 //tsTop
X = X + 100
FIN
And the same code as previous...
PROCEDURE CreationGroupe(ObjMaster,oboxpic,oboxjauge)
ogroup est un objet Automation dynamique
ogroup = :goObj>>CreateGroup(ObjMaster)
ogroup>>AttachProportional(oboxpic,5, 10, 95, 75)
ogroup>>AttachProportional(oboxjauge, 5, 75, 95, 100)
oboxpic>>LoadPicture(ComplèteRep(fRepExe())+"img\"+"USINE.BMP")
:nbgroupe = :goObj>>Groups()>>Count()
ogroup>>Tag = :nbgroupe
ObjMaster>>Tag = :nbgroupe
oboxpic>>Tag = :nbgroupe
oboxjauge>>Tag = :nbgroupe
:nbgroupe = :nbgroupe + 1
RENVOYER ogroup
Here you see, i create my box in the good order and also affect the z-order higher than 0
0 = master.