You're not alone in this, many people complain of the same thing, citing that it used to be so easy to drop 10lbs at first and as the yoyoing continues it becomes harder and harder to have that initial swoosh of weight loss that you had with previous starts. Age and childbirth have something to do with it, as do genetics and yoyo dieting.
A very controversial theory is the set-point weight. It's when you pretty much even out at a certain weight and can't seem to go over and under that amount for prolonged periods of time. The theory itself makes me cringe but it is very difficult for me to deny that there is some truth to it. Here's just one source, you may want to look into other sources too
http://www.obesityaction.org/educati...t-we-dont-know I think the article makes it sort of easy to understand and it compares it to a thermostat.
What's most discouraging is that trying to alter your set point proves to be extremely difficult because dieting is very difficult to sustain long term, thus the overwhelming number of people who lose weight and then gain it back. Yoyoing up and down has a very tangible effect not only on our metabolism but our cardiac health. What I find most encouraging is that your thermostat can be altered by changing your brain! So that's kind of what I set out to do, dieting didn't work for me, I could never do it long enough and it felt like torture. I could never adapt to restriction and always ended up binging. So I set out to literally change my brain by rewiring it. I follow IE and it's really really working. I've lost 20lbs in 6 months without trying, eating all the foods I love in the quantities I crave, I'm not bound by nutritional rules and so the weight has stayed off and it's impossible to imagine gaining weight since there's no cheating. I can totally see my personality change and feel my brain working in a different way around food.