The Bobble stitch is one of my favourite stitches.
The secret to success with this stitch is to always remember that the Bobble is made on the wrong side of your work!
The stitches either side of the bobble must be at least half the height of the bobble.
If your bobble is made with double crochet then the stitches either side should be a single crochet, if you are making treble bobbles then use half double crochet either side.
Normally a bobble stitch is worked in three rows or rounds.
First a foundation row of single crochet worked on the right side of your work, then turn to wrong side and make the bobbles with single crochet between, turn again to the right side and finish with another row of single crochet.
This photo shows treble bobble, five treble crochet are made in the same way as a cluster.
You can see the stitches either side of the bobble are half double crochet.
Also note how as the following half double crochet is made it pulls the bobble down making it pop out on the right side of the work.