DC-3 ILS onto KBOS, landing short

I have been running various aircraft into KBOS Rwy 15R, practicing ILS approaches.

So far all of them have behaved as expected & executed a satisfactory ILS down to the runway.

However, the GNS 430/530 & AP equipped MSFS/AH Retro DC-3 is picking up the glide path ok & descending, but in the last few miles it is dropping below the glide path & tries to land short.

Other aircraft with the same GPS & AP combination are making it to the runway.

I am not flying a slow approach so I can't figure out why the DC-3 is bailing out.

Any thoughts?

T45