If all of your events/actions are setup correctly, and still having issues with collisions, falling through platforms etc. Take a close look at the collision box, and the image point on all frames of your player, and other objects it interacts with.
Sometimes collision boxes without completely flat edges can cause issues. Lately I've been using a perfectly square sprite(invisible), and pin my player sprite to that invisible sprite. Then manipulate each one as necessary in the Event Sheets. I've experienced issues in the past with edges on collision boxes(especially on my player sprites) that were at a very slight angle, say .14 off from one point to another, on either the x or y axis.
I definitely recommend going through some tutorials here on the Construct website, YouTube, and other educational websites as well, to get familiar with the software. To make sure it is not something being setup incorrectly in your event sheets. Even after gaining quite a bit of knowledge, it can be tricky to find certain bugs within your projects. But, with more time and practice comes more skill and mastery, as with anything :)
EDIT: But, most of the time, weird glitches like you mention, have to do with the collision box or image points within my Sprites. Or some confliction within my Event Sheet, that takes me some time to find.