Sometimes using an abdominal sonar can be difficult at times..if the doctor cant see clearly they can do a vaginal sonar its much better. A gestational sac and fetal pole should be seen by then.
They try to encourage not to have your first dating scan until AT LEAST 7 weeks as before that it can be hard to see and also can make people worry that they're not actually pregnant or have lost/are losing the baby. Don't stress too much about it. The baby is the size of a seed, which as you can imagine, would definitely be hard to notice. Nutrition won't have much to do with the size at this stage. Its just the gestational age which makes it hard for the sonographer to see the fetus.