Mri showed excess fluid in hip joint. Needle aspiration as unable to obtain sample of fluid. Why and what does that mean?

Depends. A needle aspiration may not obtain any fluid if the needle was not adequately placed into the joint or if there is no fluid at the time of aspiration. Typically a hip joint aspiration is performed under fluoroscopy where needle placement is confirmed with injection of contrast medium (dye). For pediatric a hip aspiration may be performed under ultrasound guidance.
Depends. Sometime the fluid can be posterior (in back of hip joint) so if you come in from the front, you may not get it. Usually, you can though if there is a lot. Also, the aspiration of the hip needs to be done with image guidance to make sure you're in the right spot.