Here are possible reasons that may cause the situation:
1 Network error.
2 The sender is blocked by the receiver. You may try to contact them via other methods, like email.
3 The messages were defined as spam. On Alibaba.com platform, we have a system for anti-spam that will check all messages sent through communication tools on Alibaba.com, such as inquiries and chat messages to confirm if there is any spam.
If you send same messages many times, or send one-way messages to friends or stranger's account or send messages include spam advertisement (such as provide service to invoice, help apply business license and so on), the system will collect them for analysis.
Once the messages were defined as spam, the account will get punishment. So please make sure to use Alibaba.com communication tool for normal business negotiation and business chat, do not use it as tools for sending marketing information.