Answer: Based on your description, I am not certain what has happened to your lilacs. Lilacs are usually reliable bloomers when planted in full sun. Some varieties bloom more heavily in alternate years, so that might be a partial explanation.
The most common reason for lack of bloom is either too much shade, or pruning at the wrong time of year. Lilacs bloom on old wood that grew the previous year, so any pruning should be done right after the bloom season.
If they are overgrown and lack vigor, you could try renewing the plants by removing about a third of the oldest stems, cut them off at the ground level. Next spring, right after bloom season, cut out half the remaining oldest stems. The year after that, remove the remaining oldest stems. Also, remove any new growth that is smaller in diameter than a pencil every spring. This will rejuvenate your shrub and encourage vigorous new growth.
As far as fertilizing, each spring you could top dress with compost and/or a general purpose granular or slow release granular fertilizer per the label directions. Using an organic mulch year round will also help feed the soil. The mulch should be spread in a flat layer two to three inches thick over the root area but should not touch the stems.
To know if you need to lime or not you would need to run some basic soil tests. Lilacs prefer a pH of about 7.0 but will tolerate a range of pH. If your soil is extremely acidic, lime to raise the pH might be helpful. Your local county extension should be able to help you with the tests and interpreting the results.
I hope this helps you trouble shoot.
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