There could be a number of reasons explaining this occurrance, some of the most common ones will lead you to examine the texture of your nail bed along with the formula of your nail polish.
Starting with clean nails, use a simple buffing block, like the 4-Step Nail Buffer from Sephora (http://www.sephora.com/4-step-nail-buffer-P265712?skuId=1253335) to even out any ridges, unevenness, and rough edges from your nails. Multi-sided buffing blocks are designed with sides that have a finer grit than a traditional nail file to help gently polish down and even the surface of your nail bed and then helping to smooth out the texture in preparation for polish and color.
If after buffing and smoothing your nail bed you still realize or find ridges, try using a ridge filling base coat like Butter London's Nail Foundation (http://www.sephora.com/product/productDetail.jsp?keyword=butter%20london%20ridge&skuId=1414564&produ...). That product is also handy for giving nails a nice and natural nude tint, perfect to be worn alone or as a base for manicures.
Ridge filling base coats aren't the same as traditional base coats as those formulas are thinner and may just sink into areas of unevenness. RFBC's help smooth over trouble spots to give a smooth surface for polish to lay better on.
After painting your nail with the RFBC, apply a coat of your polish evenly and with moderation. Allow proper time for it to dry then layer on another coat if needed, waiting for coats to dry will prevent air bubbles from forming inbetween layers. Also, rather than shaking your bottles of polish, roll them gently between your palms to mix it, this will prevent air bubbles from forming in the bottle and transferring to your nail as well.
After your polish is dry, finish itoff with a top coat.
Be sure to check the texture and formulas of your nail polishes periodically, sometimes polish that is old or have been exposed to too much air can get thick, tacky, and clumpy, causing application to be more difficult and the texture to change overall. Certain formulas can be saved with using a nail polish thinner, but if it's a bad batch to begin with there's no point in trying to rescue it.
after u shake ur polish let stand for five minutes, ive found bumps are usually a result of bubbles, otherwise get plenty of paint on one side and try and cover ur nails in a few strokes as possible
I completely agree that there are air pockets in your polish! Gently roll the bottle to combine & apply several thin coats rather than 1 extremely thick one. Also, allow your polish to dry for as long as possible in between coats, as that can also cause bubbling and unevenness I've also noticed problems when I've used cheaper brand nail polish. While some brands are fantastic, I always spring for well known brands, such as OPI, Essie, Orly, etc. My manis last longer & always look flawless