Your mix adds up to 187ul? If the buffer stock is 10x, I think it should be 180, so that's a little off. I suppose if the anneal temp were right at one of the primers' Tm, the more dilute salts could push the Tm over the edge so it couldn't anneal.
Primers and taq I don't know the concentration. OK, I looked up the Platinum protocol, and the taq looks right. Primers I don't know.
Mg is right if it's from 50mM as in the protocol.
dNTPs... if you mean 20 mM, OK.
I think a colony control is always a nice idea with a colony PCR. Unfortunately, I think this after I don't actually have good one on hand. If the PCR product is cloned from a different vector, bacteria containing that would be nice.
3.5 kb is a big to be able to readily PCR with standard taq, and a mix contaminated with bacteria. If it works routinely for you, than good, but it's possible you were fortunate the first time. Ideally you would have another internal primer on hand to PCR a smaller product.