The blood is drawn from the veins and following can cause the difficulties;
1) Dehydration as you mentioned due to which the veins may collapse.
2) Technician / phlebotomist may be inexperienced.
3) Veins may be thrombosed as there may have been frequent blood draws from the same veins.
4) Vein preparation may be inadequate.
A difficult draw may be due to low blood pressure occasionally but that happens in hospitalized patients.
This may not be indicative of a disease process. So one may not be worried.
Next time, you need to ask the phlebotomist to take some time to prepare the vein properly, tie the tourniquet with an adequate pressure (too tight will block the artery which does not let the vein fill up and too loose may not let the vein fill up adequately).
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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