One of the side effects of chemotherapy is that is can be very difficult on the peripheral veins (those small veins found in your hands and arms). It can cause
scarring, making the veins difficult to find, even for experienced phlebotomists. Unfortunately, there is nothing you can do to improve the condition of your veins, as they are forever changed as a side effect of the chemotherapy you received. Take heart, though. Many people, even those who have never had chemo, are "hard sticks". It is a common issue, so, please don't worry.
Regarding the low potassium in your lab results, there are a number of reasons why this can be low. The most common reasons are medication side effects (like water pills and steroids), GI losses (from vomiting or diarrhea), or kidney problems. Since you are getting it checked by your doctor's office, if they feel it is worrisome, I am sure they will let you know if it is something they need to monitor.