That's right. Grossgrain will slip through most ladderlocks, as was noted in an earlier post. Twill tape, ditto. Certainly for 3/4" and up, anyway. And half inch buckles are really to flimsy for a pack strap, I think.
Suggest you use a good quality 3/4" or wider nylon webbing. Have never seen a webbing as thin as grossgrain, but ?
There was a thread recently where someone was looking for buckles suitable for grossgrain. Links for some exotic nifty looking buckles were provided, but don't recall that any were for small quantity retail purchases.
Note: As was also noted, it is easy to thread webbing with the buckle upside down, as some buckles look like the webbing should go that way. Turn the buckle over, rethread, and see if it makes any difference. That is probably not your problem, but wanted to cover all bases.
Best luck with the pack.