When it comes to performing sex there can be a variety of ways to derive pleasure. While some people can accurately and smoothly perform sex in all imaginable sexual postures, there are others who prefer only certain sexual positions that can make intercourse pleasurable.
Some men reach at the climax of sexual pleasure while they are beneath their female partners. But most of other men get their women on their top so as to prolong their ejaculation and help their partners reach orgasm.
It is very difficult to say why men cannot often reach orgasm while they are below their female partners but there can be psychological or physiological factors that can either delay or stop their orgasm.
So it is absolutely normal if your man does not ejaculate while you are on his top. May be he had reached orgasm in this sexual posture at some time, but now he may be feeling pressure that is delaying orgasm.
It may also be possible that your partner becomes very excited when he comes beneath you for the first time. In order to avoid premature ejaculation he may also have done everything which he could do so as to avoid too much of arousal and excitement.
By laying below you your partner may have obstructed his arousal level to avoid ejaculation. In addition to this your partner may be waiting for you to reach an orgasm first before he could ejaculate because after ejaculation it may be difficult for him to maintain penile erection that could eventually leave you sexually unsatisfied.
Therefore stop worrying and take it easy. Men usually want to have sexual intercourse in traditional missionary position and a variety of other sexual positions. But when they feel too much of arousal and stimulation they prefer to come below their female partners so as to avoid ejaculation.