What is your recovery model set at?
It sounds like it might be set as SIMPLE. If it is, try setting it to FULL and then attempt to continously add data to the database.
If it is in SIMPLE then I'm guessing, by your description, that the log is clearing itself out before you manage to fill it up. The log, in simple recovery model, will truncate whenever you hit a checkpoint. Checkpoints can occur from many different events, such as when you backup the database.
Also, keep in mind that the log may get minimally written to depending on how data is being inserted. Example if you "bulk insert" then the log may only hold enough info to reproduce the bulk insert, as opposed to holding all the row data info. You may want to ensure, only if you are intent on filling up the log file as much as possible, to insert line by line...this is probably the most intensive action that the log keeps track of.
Keep in mind that the FULL recovery model will not truncate the transaction log until you backup the log file. In other words, if you backup the database (while using FULL recovery model) the log file will not be automatically truncated like it is in the SIMPLE recovery model. This can resolve your issue of not filling up the log file for your testing....but, do be sure to keep an eye on it and backup the log file (or even shrink it, if needed) at the end of your testing. SQL will not do this for you...even if you set it back to SIMPLE model after all your testing, you may still need to manually shrink the file down; if you don't want the final log file size you are at when completing the testing. As illustrated in my article, if the log file does become full and can no longer grow to allow new transactions then you will receive the error I mentioned.
Those are just a few immediate ideas.
Hope this helps!
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