oh ok. the spike is tricky to land. go into debug mode and look at his dair hitboxes. the upper two (shoulder and head) are the spike ones. in order to land them, you need to connect with ONLY those ones. both of them can connect simultaneously, but if the next hitbox down connects at the same time (the third one down, the pop up sweetspot) then it will take priority. even the tipper hitbox will take priority over the spike. the hitboxes start on frame 7 and are active for 4 frames.
now that you can see where the hitboxes are, you know where you need to connect. i find it easiest if you connect with the later portion of the move (the 3rd and 4th active frames) while moving forwards into your opponent (ie after a dtilt). you can also more easily connect by hitting with the first two frames, but with your back to them, like during an edge guard. just depends on what youre doing
what you should try to do first is connect with it off a dtilt. as soon as you run forward, jump, and leave the ground, try to quickly press down A or c stick down, which will get the dair started so that the later frames will connect. aim to put your head right under them. you can also connect with it off a fair, but thats much trickier since is very DI dependent.