The vast majority of heat transfer between the CPU's heat spreader to the heat sink is via metal-on-metal contact. That's why you can mount the heatsink with no thermal paste and still run the computer.
The only role of the thermal paste is to fill in microscopic air gaps which form between the two metal surfaces because the surfaces are not perfectly flat and smooth. You want as little thermal paste as possible - just enough to fill in air gaps. The amount recommended by thermal paste manufacturers is almost universally way too much. They want you to use too much so they can sell you more paste.
You do not want a layer of thermal paste like you're putting mayonnaise on a sandwich. Thermal paste is about two orders of magnitude better at transferring heat than air, which is why it's used. But metal-on-metal is about two orders of magnitude better at transferring heat than thermal paste. So if you use too much paste, you'll reduce the amount of metal-on-metal contact and actually make the CPU run hotter. If you have enough paste on there for an air bubble to form, you have too much paste to begin with.
Generally I use about a half-pea sized dollop of paste, then slide and twist the heatsink around as I press it down. This spreads the paste out and the excess starts to ooze out the sides. When I start to feel metal grinding on metal, then I know I have metal-on-metal contact, and I strap it down. (The compression straps' function is to bend down small metal peaks and ridges on the heatsink and CPU. Without compression, these peaks would separate the two surfaces further, reducing the amount of metal-on-metal contact). You can do a couple practice rounds. Apply, then lift it and check out its spread, to make sure your twisting and sliding motion is producing even coverage. Then wipe it off and start over. You'll probably find that even with a half-pea sized dollop, the majority of the paste still gets pushed out the sides.
The other possibility is you've got debris like hair or dust or a grain of dirt in there. Debris will tilt the heatsink relative to the CPU, causing a reduction in metal-on-metal contact, and thus higher temps. Make sure the inside of your PC and the two surfaces are clean before you apply paste.