Unreliable DTMF on bridged SIP call

I seem to be experiencing unreliable DTMF once a SIP call has been bridged. An inbound call is bridged to an outbound call to an IVR system. Once bridged and in the IVR, all DTMF has to be entered really slowly, if keys are hit too fast they are ignored. Dialing the IVR direct works fine so I know it’s not a problem there.

Anyone see this before? Using RFC2833 on both end.