Something else.

The lactic acid build-up theory was discredited earlier in the century.

http://www.nytimes.com/2008/02/12/he...musc.html?_r=0

http://www.livestrong.com/article/51...ter-a-workout/

My guess, based only on my own personal experience and the very limited information you've given, is that you need to do some training, and keep at it.

Hills and sprints take work. When I just ride along, I quickly lose my sprinting and climbing abilities. Within a month, calls to the engine room are answered with giggles and burning. I have to keep up with it.

Three-quarters of my riding is commuting. While I prefer my long loops to work because they let me stretch out and crank, I have to mix it up. If I stick only to the long loop, I can rack up impressive miles. (As I did in June, my third highest mileage month.) But I suffer on the weekends when rolling hills come into play on club rides. And forget sprinting.

I have to use my hilly route, which is a series of short, steep, punchy hills chained together. I have to take my short, stop-and-go route to race the cars for my intervals.

In spring, after commuting on my short stop-and-go route all winter, I can probably out-sprint Mark Cavendish. This time of year, not so much.

In late August when training for a very hilly ride, I use my hilly route twice a day for a few weeks, and do hill training again on the weekend. Then on that very hilly ride, I can climb like a goat. I lose that pretty quickly again afterwards when I go back to miles-based riding.

So it's the age old story. If you want to get better at climbing hills, climb more hills. Same for sprints. And same for distance. If you want to be good at all three, you have to practice at all three and keep up with it.