Anatomy question

Just had my check up appointment and asked about the ultrasound scan that I have on the 10th. I asked what we should expect as far as what they look for and how we were also excited to find out the gender. They told me that the purpose is to make sure everything is ok and “sometimes” the gender is obvious but not something they take time to find specifically. They also don’t call us back in later if they can’t find it that day. Is this normal?? I have a party planned for the following weekend and now I feel like I’ll only be lucky if I find out gender.