Could you provide the output of the following?
get alg pptp
get alg pptp counters
get alg pptp xlate
get rm group active
get nat cookie | include allocated
unset ff
# Repeat above command until you get "invalid id"
set ff src-ip <x.x.x.x> dst-ip <y.y.y.y>
set ff src-ip <y.y.y.y> dst-ip <x.x.x.x>
# <x.x.x.x> is the source IP address.
# <y.y.y.y> is the destination IP address.
# There are multiple options available for filters. Select the best filter for your situation (ports, protocol, etc.).
debug flow basic
debug rm all
debug nat gate
debug nat xlate
clear dbuf
# <initiate the traffic you are having issues with>
undebug all
get db stream