The daily obligatory Prayers are obligatory to be performed in its time, and the recommended Nafilah for every obligatory Prayer is just recommended and not obligatory. Recommended prayers are very useful and get us nearness to Allah (SWT) but hey are not obligatory on us. There are narrations that recommended prayers are obligatory on the Prophet (SAWA).
We need to try our best to perform the recommended prayers as much as we can, to gain frm the great benefits of it and to be more near to Allah (SWT). In Hadeeth Qudsi that Allah (SWT) says: My servant keeps on performing recommended prayers till I love him, and when I ove im, I will be his ears with which he hears in, and his eyes with which he sees, and hand with which he acts.....
Recommended prayers are not Wajib that if we leave it like we have left an obligatory, but it is very much helpful in our spiritual uplift and we should never ignore trying to perform the recommended prayers as much as we can.