Patchy foil is the single most common message I get, and in four years I have never once seen it caused by the foil itself. It is always heat, pressure, toner or card, and every one of those is fixable in a couple of minutes. Work through this list in order.
First, look at where the patchiness is
The pattern tells you the cause before you touch anything.
Patchy in the middle, bright at the edges: uneven roller pressure. Skip to step 3.
Patchy everywhere, foil looks dull or speckled: not enough heat. Start at step 1.
Foil missing from thin lines and small text but fine on solid areas: not enough toner. Go to step 4.
Foil sticking to the paper around the design as well as the print: too much heat, or the card has a coating. See step 5.
Foil lifts off in flakes after it looked perfect: the card surface. See step 6.
Step 1. Let the machine get properly hot
The ready light on most laminators comes on before the rollers are at full temperature. Give it another three to five minutes after the light. If you have been running sheets back to back, the rollers cool with every pass, so pause between sheets. This alone fixes about a third of the patchy transfers people send me.
Step 2. Use the thickest setting and go through twice
Set the laminator to its highest thickness or temperature setting, even for thin card. Run the sheet through once, do not peel, turn it around and run it through again the other way. The second pass catches everything the first missed and evens out any cool spot on the rollers.
Step 3. Add pressure with a carrier
Fold a sheet of ordinary printer paper in half and sandwich your card and foil inside it. Then add a second sheet of thin card on the outside. The thicker the sandwich (within what the machine will take), the harder the rollers press. If the middle of your design is still dull after this, your rollers are bowed and no setting will fix it; that is when I suggest a four-roller or dedicated foiling machine.
Step 4. Give the foil more toner to grab
Foil only sticks where toner is. Thin lines, light fonts and grey areas have less toner, so they foil poorly. In your printer settings turn toner density or print quality to maximum and switch off any toner-saving mode. Make thin lines a little bolder in your design. If you are printing from a home laser and the black looks slightly grey, print the same page twice, lined up exactly, to double the toner.
Step 5. Check the card is not coated
Glossy, silk or "photo" card usually has a coating that goes soft under heat and grabs foil everywhere. If your foil is sticking to the blank paper around your design, this is why. Switch to uncoated smooth card. Our 330gsm smooth black card is the surface I test every foil on, precisely because it is uncoated and dead flat, so the toner sits on top and the foil has nothing else to grab.
Step 6. Peel slowly, and peel warm
Take the sandwich out and peel the foil back within about ten seconds, in one slow, low, steady motion, pulling it back over itself rather than up. Ripping it off quickly or peeling when it has fully cooled both cause flakes and missing corners. If a small spot is missing, lay a fresh piece of foil over just that spot and run it through again; the second layer will fill it.
A note on finishes
Metallic foils are the most forgiving and the best place to start. Holographic and satin matte foils have a thicker top layer and need the machine a little hotter, so if a metallic works but a holographic does not, it is heat, not the foil. Patterned foils behave like metallics.
If none of that worked
Send me a photo of the patchy piece, tell me the laminator model, the printer, and the card you used, and I will tell you exactly which of the six it is. I would rather spend five minutes on that than have a roll sit unused in a drawer.