It would help if you could share your project file or a screenshot of your event sheet.
If collision is not registered, objects must be moving very fast and be quite small.
Say, if two sprites 100px wide are moving towards each other, their combined speed should be 3000-6000px/sec or more.
You can make a simple event to check that the collision is indeed not registered:
Sprite1 on collision with Sprite2 -> Browser log "collision"
If this is really the case, try increasing their size by pinning a bigger collision box (invisible sprite) to them. You can only use it to detect collisions while objects are in the air and moving very fast.