Motorcycle Questions? Ask a Mechanic.
This issue can be caused by either an improper ignition timing or improperly adjusted pushrods. It can also be caused by mechanical issues inside the engine or inside the primary drive. This is assuming you don't have electric compression releases installed that are no longer working for some reason.
You can verify this by reaching down in the center hole of the rocker box cover and feeling for a rubber cap. If there is a cap present you have them and they are likely no longer working. If you don't feel the rubber cover you likely don't have them.
The first step is to verifying ignition timing for the ignition system you have installed.
Once that is verified correct, S&S recommends adjusting the pushrods so that a .050" oil cushion exists in the hydraulic lifters. This allows the valves to crack open earlier to bleed off excess cylinder pressures earlier. I recommend checking the cylinder pressures first to have a reference point. Anything over 180 psi is difficult for the starter to push past.
I hope this helps Please let me know if I can be of further assistance. Thanks, P.J.