If your cockatiel is showing signs of falling, not being able to perch, or overall weakness, you should seek avian veterinarian attention as soon as possible. A cockatiel that does not perch or loses its balance is a sign of a sick bird.
"Infections, undesirable high toxic levels in your cockatiels body like lead, tumors, and lack of certain vitamins can make your bird lose its balance."
If your cockatiel has recently injured herself by crashing against a mirror or a window, it might be suffering from head trauma, vet attention is required.
Kitchen fumes can also make your cockatiel sick, and lose its balance, it takes very little for a cockatiel to die from kitchen fumes, specially if your cookware is made with teflon.
If your cockatiel is taking prescribed medication from your vet and started to lose its balance after the medication was prescribed, stop the medication and call your vet, it might be and undesired side effect.
As always the information offered here is to provide guidance and is not intended to be a substitute for the good advice provided by your own avian vet. When in doubt always consult your own veterinarian.