It's not very easy to say what you are doing wrong while bowling in indoor cricket.
As a person who has never played indoor cricket, I cannot pin-point any differences. However, while I play with leather balls and tennis balls, I notice that there is a marked difference in batting and bowling techniques.
Timing the ball is of utmost importance while batting in leather ball cricket and hitting the ball hard is optional, as timing can get the ball to the boundary with very little effort. However in tennis ball cricket, timing is obsolete and power is everything as the ball is very light comparatively and hardly has any momentum while travelling forward compared to a leather ball. I guess the same goes with indoor cricket, as there isn't a huge distance for the ball to be covered.
The adjustment I try to make during bowling with a tennis ball is to knock off the pace for beginners, which forces genuine batsmen to use all their power in getting the ball over the infield or to the boundary. To tail-enders and players who find batting hard, I bowl with full speed which gives them less time to adjust to a particular shot, thereby increasing my changes of getting a wicket. The other adjustment is with the length. As there is no swing with a tennis ball, I drop try to pitch the ball on the short of length spot. This is basically to prevent the batsman from freely swinging his arms to a length/half volley ball. However, the margin of error in tennis ball is very less. Bowl a little forward, the batsman swings you over long-on/off. Drop the ball short and the ball literally stops and stands to be hit over cover or mid-wicket or hooked to the square-leg boundary. Apart from this, variety (quicker/slower deliveries, off/leg cutters, etc) also keeps batsman guessing as to what will come next and increases a chance of a dismissal.
Considering that the balls used in indoor cricket are lighter than the actual leather balls used in outdoor cricket, the above tips might be of help. It takes a bit of practice and knowledge of the places where you can pitch the ball which can help you as you go along. This generally comes with experience. So, I guess the answer would be to play and learn as you go along with each practice session or match and thereby fine-tuning your bowling strengths to indoor cricket bowling.