There are various reasons for plants not flowering. Here are three reasons (which is a start)

1. Pruning at the wrong time of year - therefore the growth containing a season's flower buds pruned off and insufficient time for a plant to grow more in time for its normal time of flowering

2. Severe pruning/regular pruning - much the same result, as a plant cannot produce enough ripened wood in time to flower before it is pruned again. E.g. some flowering shrubs produce flower bud on the second year's more mature wood. Some hydrangeas are an example, and need 'tidying up' rather than pruning.

3. Plants cannot produce 'ripened' wood because they are grown in too much shade.

W.

Patience. Many plants do not flower immediately (or even soon). For instance, Clematis forms make take quite a few years to make a decent plant and flower as expected.