Why am I only allowed to train one troop member at a time?

Check you have not locked the number of troops you can train - there is a little padlock icon next to the minus ( - ) symbol when selecting how many troops to train. Check you haven't accidentally pushed this.

Check below the slider for selecting the number of troops to train. It will show the resources needed to train (food, wood etc). If you don't have enough it will restrict what you can do. If the numbers are red then you can tap them to go see if you have stored boxes of the resource that you can use to add to your total and let you train.