First law of parsnip growing states that if you sow 3-4 seeds to each station, some stations will have 100% germination, others none.
I have successfully transplanted seedlings to fill the gaps, but they need to be no bigger than first true leaf stage, and great patience is needed to ease out the long fragile roots without damage, and equal care to replant without kinking them.
Having sandy soil helps, but an alternative is to transplant into pre-dug holes filled with multi purpose compost (I actually sow them using that technique anyway - as well as being sandy my soil is also quite stony.)