Question

why for blood test sometimes we wont be able to draw it from the veins/arteries?/

Answer 1

Bloodpresure is Arteries is far above those of veins. When extracting vein blood, you might notice a small bleed afterwards but it is insignificant to that of a Artery.
It would be hard to stop the bleeding therefore medical personell avoids Arteries, the only exception that i know of is capillaries tests, MOST of the analaysis which can be done on the extracted vein blood can be done on capillary blood, this is used on people either afraid of needles or those whom suffering from primary or secondary bloodloss.

To summarize. Arteries has around 90+ mmHg depending on your bloodpreasure (force of which the blood leaves Ventriculum to the Aorta. while veins usually have between 5-8

Answer 2

*Noticed a typos but im in a hurry, Im sure you'll figure them out.