IR photodiode doesn't show 38kHz carrier wave on oscilloscope
Probably the photodiode is acting (with the 'scope input capacitor) as a rectifier for its own signal. 

Photo current will only flow one way from the photodiode. This will charge up the input capacitor of the 'scope. When no light is present, the charge will only bleed away slowly through the scope's input resistance, which is maybe 1 or 10 megohms. 

Apparently the bleed is fast enough to let you see the data signal, but not fast enough to let you see the 38 kHz carrier.

As for a solution, try adding some lower-value bleed resistor (maybe 1 kohm), but realize this will likely also reduce the signal amplitude, maybe too low to measure. If that's the case, you'll need to build a real bias circuit or even a transimpedance amplifier to condition the output of the photodiode before measuring it.