Program upload issue

LED on pin 13 of 328 of arduino module blinks before uploading. the moment uploading starts it stops blinking. but few of the board it keeps on blinking even while uploading the software and uploading fails. what could be the reason ?

Which Arduino module? Blinking at which rate (quick or slow)?

Usual sequence of events:
A upload starts with the reset of the board; that should give two or three flashes on the built-in led. After that the built-in led should be 'quiet' till the sketch is loaded. Once the sketch is loaded, the built-in led might or might not flash depending on the code that you loaded. E.g. with blink the built-in led shoud be on for one second, off for one second etc.

Forgot to ask in in previous post.

Which OS? What does it think about the board (Windows device manager, Linux (and possibly Mac) lsusb and dmesg)?
Which error messages?

Ports might change; did you select the correct port.

no error message appears, it just says as uploading and continues to blink. out of 20, 3 to 4 nos are not uploading the sketch in the same setup.

arduino pro mini module 16Mhz, 5V, window 10,

One second off, one second on? If so, looks like the reset does not reach the 328P; sounds like bad boards in my opinion, ship them back.

yes, that what i felt. but thought if something can be done. anyway thanks for the reply and time.