It's normal, the 737 wing is too low: the larger tanker requires a refueling point at least 3.40 mt. high, and the hydrant at least 3.00 mt.
The smaller truck doesn't have an animation, this might change in the future. However, since the 737 is not that low, it might be enough to just move the refueling point further out on the wing, and you should be able to get the hydrant (only at Gates), which has the best animation.