Tension. I have recently sewn two louses and started on a third,however,the tension is completely

Question

I have recently sewn two louses and started on a third,however,the tension is completely off whack!Noone has messed with it and the fabric is the same...I have spent the better part of today trying variations of tension,stitch length,etc and still I cannot figure it out. The top stitch is either fine and the under stitch is all "loopy",or the top stitch is too loose and the bottom stitch is fine....HELP!!!!
The sewing machine is a Brother xl-2600