Blood draw elderly tips.

Hey y’all so I have a question on my hard sticks. I’m a MA of a primary care place with almost all elderly pts. Most phlebotomy work is just standard basic labs CMP, CBC. Sometimes I get a full Tiger tube but then when I switch to the lavender tube the blood stops and/ or vein blows out. We only use butterfly needles. I’d like to say I have a really steady hand and not moving the needle when switching tubes. Is the a reason why this sometimes happens? Is it normal when the demographic I do?