[2+2=421]

Just had my engine rebuilt, 3 months back, maybe 300 miles and leaking from the rear main seal. Machine-shop used 1 piece viton seal. I asked them what the deal is and i get.. well sometimes the never leak and sometimes they do.. what is the machine-shops responsibility in this? I know they wont pay to r&r the engine, so whats standard protocol?
Also when i replace it do i use a two piece seal? Not interested in removing the crank etc., very frustrating!!