Easy fix kid. Get some boraxo, like at a laundry mat. Mix 1 (one) level table spoon of the boraxo with a gallon of water and pour it around the drip line of the plant. Boraxo has this trace element called Boron that is essential to plants to flower and make seed etc. How ever Boron is extremely toxic in its pure form and can sterilize the ground for several years. This is why I tell people to use the Boraxo soap and then only a tablespoon. You only have to do this treatment once every 5-7 years for you will have and abundance to satisfy the needs of the plants for a long time!!! To anyone else this will work on fruit trees as well. BUT and I repeat But do not use more than the one tablespoon to a gallon of water; follow this as if God Himself were talking to you. The damage will be irreverseable if you take on and American attitude of “More Is Better”!!!!
3 factors immediately pop into mind… Is the Hydrangea getting the proper amount of light? Is it in an area that is too shady? Do you prune the Hydrangea? Some Hydrangeas bloom on old wood, so improper pruning could be causing it not to bloom.. What zone does your MIL live in? Does she protect it from frost? If the Hydrangea is in a shady area, then it could succumb to frost every Spring.
Maybe it’s too much sun then? The afternoon sun could be stopping it from blooming. That happened with the lobelia I planted this year. The nursery tag said it would do well in sun, but the afternoon sun was too much for it and I had to move the plants before I lost them all. Here are a couple of excepts from www.hydrangeasplus.com: Sun can also hurt the blooming ability of a hydrangea. Too much direct sunlight drains the plant of moisture. If the foliage is still green and healthy, the hydrangea is most likely using all the water to retain its health and the bloom is left with nothing. The best environment for most hydrangeas is morning sun and afternoon shade. You may move hydrangeas as well.
Be sure the plant is well established before trying this method as a strong root base is essential. Once the plant is finished blooming in the fall before it’s dormant, prune the hydrangea. Tie the branches together with twine to prevent damage. Dig up the roots of the hydrangea and be sure you dig a large rootball containing lots of roots. Replant the hydrangea immediately and water well. You may also divide a hydrangea using a sharp spade. Colors may be slightly different in the year following the transplant.