I tried to send a message and found that it has one mark. I can see the last seen message, which means that it was not yet seen. Should I try to send the message again?
One checkmark means that the message has been delivered to Whatsapp servers, but has not been delivered to the recipient's phone. You don't need to send it again, the message will be delivered to the recipient the next time his/her phone connects to the Whatsapp servers, assuming you're not blocked. Sending it again will not help, since it's already delivered to Whatsapp servers.