RabbitMQ is losing messages

I have just installed RabbitMQ, I'm using it with logstash. I have been sending messages but I lose a lot of data. I have no idea how could I solve it. I'm using the default configuration in rabbitMQ.

These are the output in logstash and the input in logstash:

output {
rabbitmq {
host => "IPHOST"
exchange => "logstash-rabbitmq"
exchange_type => "direct"
key => "logstash-key"
workers =>4
durable => true
persistent => true
}
}

input
{

rabbitmq {
host => "IPHOST"
queue => "logstash-queue"
durable => true
key => "logstash-key"
threads => 8
prefetch_count => 100
port => 5672
user => "test"
password => "test"
ack => false
exchange => "logstash-rabbitmq"
}

}
I'm using RabbitMQ Management in order to see the evolution of my queue. When I send 10000 messages I only receive less than a half. Is there any parameter I should change to imporve the behaviour of rabbitMQ? I was going to use it in order to don't loose messages but I'm loosing even more than when I didn't have it.

thank you