This happend with my DD. When she was a newborn she loved being in a sling. I had a wrap and a Mai Tei. Then when she was about 3 months old she seemed to hate it. She would cry every time I put her in one, So I didn't use them for awhile. When she was around 4 months I started using a RS with her and she loved it. We still use the RS the most, but she likes the MT, too. We havn't tried the wrap again because it's so hot.
Anyways, you might want to try out diffrent carriers and if your baby still dosn't like them, don't give up. Just keep trying every so often.