Chagrin:
You're sending pulses for 200 microseconds, each pulse 26 microseconds, so that's 8 pulses. You then similarly delay for another 200 microseconds afterwards. From the Vishay datasheet: "After each burst of length 6 to 70 cycles a gap time is required of >= 10 cycles". Per the spec you need more gap time.
Thanks! This is all very helpful. I'm very new to electronics and Arduino so just learning as I go. I had a feeling it had something to do with the duty cycle. I think you're spot on.
This is a dumb question, but just to clarify: a cycle in this context is one 26 microsecond "pulse" (i.e. one full 38kHz wavelength)? So I could send 8 pulses (208 microseconds) and then pause for >= 260 microseconds and I should be fine.
Also, I do think that timers would make more sense than using an Arduino but since I'm new to electronics I opted for something more in my comfort zone at the moment. And if I'm sending complex IR codes, I will need to use an Arduino. For the beam breaks, timers would likely suffice. I just don't know how they work yet.