Sorry, I meant a huge piece of DNA is to be amplified. The source is Cow DNA, and yes, I have triple checked all the primers and they are all fine.

Use the long range forward primer and combine with a new designed revers primer so that a product for example from 800 bp results. Then make a normal PCR for that (normal enzyme like Taq, not with the long range one) The same for the long range revers primer with a new forward primer. If you get a result, you can be sure that your long range primers work.

So you can exclude missmatch etc. from your long range primers, if you get no result, design new long range primer.

Another idea: try a temperature gradient for your long range PCR. Maybe the annealing temperature is to high/low. Did the primers nearly have the same melting temperature?

Depending on what you actually want to achieve, maybe overlap extension PCR would help? This would let you amplify your DNA in pieces which would then be joined.

Have you tried amplifying the fragment with say the forward primer and then an internal primer to give a smaller product? If the DNA is degraded then it may be why you are having problems amplifying something so large. You could try doing smaller fragments and see if there is a size where things stop working.

You have to remeber that while you are doing long PCR you have to always consider the errors that will be introduced during the reaction. The longer product the more incorrect bases are inserted. Even if you do the PCR with Pfu Pol. Consider doing the analysis on 2-3 shorter fragments. This will be easier for cloning, as well.