I can only precum and not cum

What happens when you feel like you'll climax?  Do you have an orgasm?

Ejaculation and orgasm are different. 

Are you on any medications?  Have you ever had a "wet dream" or nocturnal emission

If you are orgasming but not ejaculating, you may be experiencing what is termed "retrograde ejaculation."  This happens when the sphincter to the bladder doesn't function properly, so that semen which should be ejaculated out of the urethra at the time of orgasm instead gets shunted into the bladder and is released later, mixed in with urine. If you think this may be the problem, contact a doctor to get his opinion and advice on how to proceed.  On a different note, congratulations for keeping an eye on your penis health; too few men pay enough attention to this important aspect of their health. Regular use of a quality penis nutrient cream (health professionals recommend Man1 Man Oil) can help with overall penis health. Look for one that includes a knock-out antioxidant like alpha lipoic acid, as well as a range of viatmins (including A and E).  Good luck!