The reason your signs were taken depends on various factors. 1) What area of town did you place them in? The nicer the area, the more quickly they'll be taken. 2) Where did you place them? Medians, corners with active businesses, and residential areas are "no no's." Don't put them too close to sidewalks or curbs either. 3) Were they placed in such a way where the person taking them had easy access to do so? Don't make it easy on those who might take the signs. If it's inconvenient for you to put them out, it'll be inconvenient for someone to take them.
The summary is to put them in high traffic high visibility areas that are difficult for sign takers to access. If it makes you feel any better, I had the same experience when I first started using signs, but I developed a detailed strategy over time. If you'd like lots more helpful info on signs and a lot of other ways to find deals, I welcome you to register for our next live Wholesale Real Estate Investing Class or to order the Wholesale Real Estate Investing Home Study Course from our REI Store. If you've got more questions, please feel free to give me a call at 817-300-1132.