After finishing any type of hand sewing, I use a small dab of clear nail polish right on the knot to keep the knot/thread secure and from unraveling.
That sounds worth a go.Marg from England.
Making a Dog Poop Bag Dispenser
Making a Thread Catcher?
Making Fleece Socks
Using Thread to Untie Knots
Crocheting With Sewing Thread?
Making a Rag Knots Chewing Toy