It can be both, actually. The instrument can fail (especially the reed) and it would be hard to track that if you’re not already keeping a frame of reference on whether the instrument has been performing or sealing well lately. Make sure to have your oboe adjusted every 4 to 6 months for the best performance possible. If you’ve been taking good care of your instrument, however, then it’s definitely your skills being rusty and out of practice! Time, then,  to improve your oboe technique and get down to some regular practice!