Wow, lots of responses!
Sure, you guys are right and I need to practice more. I've been playing for quite a long time (20 years, gosh), but I usually just write and record - ie I create a drum groove, noodle around until I've got something I like and then record, then record some guitars to it and rarely ever play it again, unless I later feel that I can improve the recording... I just like writing and recording music a lot more than playing it... as a result, I progress slowly as a bass player, but my ability to write songs has improved.
But recently I started a band and began rehearsing and practising my songs... This is how I noticed those inadequacies, and was wondering if other people experience this in the same way, but judging from all the responses, it seems it's different for everybody, which is interesting.