In WPA Networks every connection is encrypted with a uniq key. The PSK is only there for authentification and encryption of the initial process of key generation for the connection. Because of this you can't decrypt traffic by only knowing the PSK you also need do capture the 4-Way-Handshake of everyone connected to be able to decrypt their traffic. 

What you are currently able to decrypt is broadcasted traffic only. This traffic can't be encrypted with the connection specific keys therewith everybody connected to the AP needs to be able to decrypt them. This is why you are able to read these packets content.

Also it's not that easy to usefully sniff encrypted wireless traffic. When you are connected to a WLAN and a packets checksum is bad your operation system will ask the AP to resend the packet until you receive it in an ideal state. You can't do this when you are passivly sniffing the traffic. So it very often happens that you miss some part or a whole conversation due to uncorrectly received packets. You need a good antenna and a good signal quality. 

// I suggest you to use `airdecap-ng` to decrypt wireless traffic instead of wireshark.