The fact that you were able to have sex means that there is no problem with your apparatus. Nature has designed the male genital in such a way that it can be erect for sometime basically to produce offspring but we want endless pleasure Prolonged erection can be harmful to the penis and hence it tries to get rest  This is not weakness If you are little lax while love making the penis tries to take rest and for safety it will not erect for some time. Again while you are fully aroused you will be able to have good erection and have sex. Don't try to go through sex when the penis is not fully ready. Or while having sex if it becomes soft wait till it is hard again, or else your body will get adjusted to lower pleasure