It is because you are practicing more reading and writing than listening and talking probably.
Here are some tips for you to practice and improve your spoken (oral )English:
Observe the mouth movements of those who speak English well and try to imitate them.
When you are watching television, observe the mouth movements of the speakers. Repeat what they are saying, while imitating the intonation and rhythm of their speech.
* Until you learn the correct intonation and rhythm of English, slow your speech down.
If you speak too quickly, and with the wrong intonation and rhythm, native speakers will have a hard time understanding you.
Don't worry about your listener getting impatient with your slow speech -- it is more important that everything you say be understood.
* Listen to the 'music' of English.
Do not use the 'music' of your native language when you speak English. Each language has its own way of 'singing'.
* Use the dictionary.
Try and familiarize yourself with the phonetic symbols of your dictionary. Look up the correct pronunciation of words that are hard for you to say.