Unfortunately, I am not familiar with the specifics of the online version. I use the desktop version. Often a locked form is some little glitch, such as paid medical on a T4, without answering the question about medical and specifying the dates ie Jan 1 to Dec 31. It is easy to miss. In most situations a missed question will fail to pull up the required form, but in the situation I mentioned you would not necessarily notice.
Some T4s also have boxes that will trigger data into a self-employment T2125, such as box 48. Again there would be information missing on the T2125 that would cause it to lock.
Sorry I can't be more help.
