Dear guy,
Please modify archicad version you are using, and check some following points may will help your problem:
1- check the layer ( these wall same layer or not) it different layer please check the number of layer
2- check the story of wall ( should be same story)
3- check the number of wall in that position ( if there are 3 wall 2 wall will connect automatically and the last one will not connect
Cheers