Don't worry, when they do the 18 wk ultrasound they need to measure all of bub's bits so they can check everything is on track, so because they couldn't get a clear look that is probably the only reason they want you to come back - they just need to be thorough. I almost had to go back again after my son's ultrasound as he was lying funny but luckily he moved and the sonographer could get all the data he needed.