Do you find that when you bend the strings, the fingers you are using tend to flatten before you complete the bend? I've seen this happen with students I'm teaching. Its often a case of them applying force from the lower part of the fingers or the hand, while the fingertips take the weight of the hand against the fretboard, causing the fingers to bend inwards and the tips to flatten.
Its important to apply force from the tips of the fingers, whilst the hand should support the fingers, not put pressure on them. This will take some time to get used to and build up strength in the fingers, but it is a good idea to start with small bends and gradually build up. Legato exercises will also help build strength in the fretting hand.
