I am finding an issue when serging clothing – when I serge a crotch seam for example – or cuffs to sleeves – my left needle will skip.  I end up with an unchained piece of thread.  Once past the ‘bump’ my serger takes off again and works fine.  I am using 4 thread stitch.  I then have to go over that area with my regular sewing machine and sew where the left thread should have been.
I contacted a repair shop and was advised this is normal – the fabric junction is too thick.  “If the serger works well on regular seams there is nothing wrong with it. “
I am using a Bernina 800DL.  Is the repair man correct – this is expected?  I did read a post where a sewer said she kept a hammer beside her machine – and pounded the seam joins as flat as possible then serged and had no issues?  
Thanks for any thoughts you have.