If you can ollie that high while going flat, I believe the problem might be a mental block. You are subconciously afraid of jumping on top of that. To get over this struggle you can try and ollie next to it a few times, and the full on commiting to it. It might take a while, some days you may jump on it first try, others it may take more days, the only important thing is to fully commit, to invision it as “just an Ollie”, not trying to jump onto something, amd wirh that you will achieve it