Evan, maybe the paint needs to cure longer before taping.
For masking for varying shades of metallic, ensure the paint has cured for at least 3 days before masking.
To test, smell it!
If it still smells distinctly of 'paint smell', then leave it a couple of more days. When ready, lightly rub a clean finger over the surface, and inspect both the finger, and the paint. If there are no obvious signs of paint on the finger, in the form of even just a slight sheen, and if there are no blemishes on the paint (from the finger), then it should be hard enough to mask.
If it is, then give the model a very light rub over with a clean, soft cloth, to remove any remaining loose metallic pigment.
Ensure that you remove most of the tack from the Tamiya (or similar low-tack) tape by employing the method previously described.When running the tape between finger and thumb, exert enough pressure to ensure that the tape has to be pulled against this pressure.
(Sticking the tape to the bench surface, or any other flat surface, only removes a tiny amount of tack, and also runs the risk of picking up unseen detritus, and / or even the very thin, virtually invisible, 'skin' layer of varnish from polished wood, or any grease or fine dust from other surfaces.)
Whenever possible, measure the area or panel to be masked, and cut the tape accordingly. Place it lightly on the area to be masked, and lightly press the edges in place all around. Ripples or creases in the central area of these masks can be ignored, as long as the masked area is covered, and the edges 'square'.
Spray the required area, keeping the airbrush at 90 degrees to the surface as much as possible, to avoid the possibility of 'cutting' under the mask. Spray lightly and quickly, passing over only the edges of the mask, misting the coat(s) in these areas, and continue to spray as normal elsewhere.
Don't worry about the possibility of an uneven effect around the mask(s) - if misted, it shouldn't show too prominently, and, it's supposed to be metal - there would be tonal variation due to reflection, grain of the alloy, and the orientation of each panel. (Once masks are removed, it's the contrast which is important, and this will reduce the effect of any initially perceived unevenness.)
As soon as possible after the paint has dried, lift each mask, at a convenient edge, with the point of a scalpel blade, and remove carefully, maintaining tension on the released part of the mask, until the mask is free.
Once the paint has fully cured (see above), lightly rub over with a clean, soft cloth.
Once finished, if desired, polish with a polishing powder, varying the amount of powder to achieve further varying effects, and ensure all powder is polished/removed from the model, before giving a further 'dry' polish with another, clean, soft cloth. (see the Bare Metal Guide I posted some time ago, for more info on bare metal finishes.)
Hope this helps.