Smoked porter

[BeestonHop_John]

hi all, I thought I'd give a smoked porter a go. Here's the recipe I'm using:

Mash (26 litres liquor; mash for 1 hour @ 65 degrees C):
Pale malt - 4.5kg
Rauchmalz Bavarian Smoked Malt - 700g
Black malt - 300g
Crystal malt - 200g
Munich malt - 200g

Boil (26 litres liquor; boil time 1hr)
Hops WGV (alpha 6.2%)
10g at start of boil & then 10g additions every 10 minutes (60g in total)
10g WGV at turn off
1tsp of Irish moss for the final 15 minutes

Yeast - Safale04

 

[BeestonHop_John]

hi all, I brewed this up yesterday, but the OG was much lower than I was expecting. It was 1038, which seems low to me for this amount of grain (I imagined 1045+).
I sparged 10litres, which made up 25-ish litres for the boil, and it was reduced to 20 litres when I transferred to the FV.
Any suggestions as to why the low OG?
(Maybe should have mashed for longer?)