The oekaki isn't meant for mobile use, so it can be iffy if its even going to work or not, much less work correctly. And since mobile devices don't support java, you can't switch to chibipaint to see if that works any better. There aren't any real fixes for mobile issues on oekaki as far as I know. You may want to try checking out this thread, its generally the place to ask about oekaki issues;
Forum/viewtopic.php?f=6&t=1941510Someone else might be able to offer another suggestion, I don't have much personal experience with using the oekaki on mobile, however from what I have seen it does seem to just not work well for some and work fine for others.