Well, @janLo has reported that his hardware needs a certain send delay between commands to make it work and he implemented a send delay feature (docu). This send delay feature you cannot use when you call pilight-send from the command line. Your issue of not being able to send alphanumeric ids has been fixed but needs testing before release. Thus soon you do not have to use the command line.
Another reason might be that your receiving hardware is not too happy to receive a 433 MHz command directly after another (let’s face it 433 MHz devices are cheap and that for a reason ;-)). You can maybe make it more happy by repeating you command more often, like all 433 remotes do.