Camera lens. I bought on ebay a Sigma 90mm f2.8 AF macro Minolta mount lens. It works

Question

I bought on ebay a Sigma 90mm f2.8 AF macro Minolta mount lens.  It works perfectly okay on my Minolta Maxxum/Dynax 7 film camera (so there doesnt seem to be a malfunction in the lens) but will not function in automatic mode on my Minolta Maxxum/Dynax 5D digital SLR.  I cannot understand why not.  I have another AF Sigma lens (70-210) which works in automatic mode on my 5D.  I have contacted Sigma and the ebay seller but neither can help.  Please can you advise me.

Thanks,

Martin