I find it interesting that the word carnal is used four times in these eight verses and the word spiritual is used only once. The word carnal seems to be used by Paul as under development and the lack of maturity. Every Christian is spiritually a baby when they accept Christ. When they are babies milk has to be used for a certain period of time. Then after a certain period of time growth begins and then more solid food can be given. The same is true in the spiritual realm. Paul's argument here is the same. "I have fed you with milk not with meat because you were not able to take it." They had an under developed digestive system spiritually.
Carnality shows up in many ways. They were choosing their favorite men to preach to them. Preachers according to Ephesians 4 should be leading Christians to a deeper maturity so they eventually would be able to discern things for themselves and teach others also.
Paul goes on to say, "We are not rivals but fellow workers." There are some that plant, meaning bringing souls to Christ, and some that water, meaning maturing the believer. It is God who gives the increase. However, preachers can do both. When I lead someone to accept Christ, I like to stay around and bring that person to maturity. It is not always possible however.