Wemb

This will happen if you go straight up and then straight down again - or anything remotely close to it.  
When you re-enter from orbit, remember, your mostly going sideways - so you get to spend a long time in the atmosphere to loose speed.  Sub-orbitally, the horizontal component to your velocity will be much less - which means you'll have less time in to the atmostphere to slow down.
There are a few approaches to this - you can try and give your re-entry body improved lift characteristics so that you glide back rather than falling like a rock - which will prolong the time in the atmosphere. Alternatively you can researcha and use drogue chutes which aren't enough to land with but can slow you down enough to deploy the main chute.
But, early on, your best bet is to simply do less steep sub-orbital missions.  Burn sideways more - you'll limit your potential Ap, but at this stage all you're trying to do is hit >70km before coming back - so use any excess thrust to get sideways velocity which will protect you on the way back down.
Good luck
Wemb