Sure, frustration is also a reason for him. He gets frustrated way too easily on bad days (like so many other players), if he has the feeling that nothing is going into his favour or if he has an imaginary problems with his racket or with his grip that get into his head sometimes and all these things can affect his play and temper, and as a consequence he is going down too quickly too often for his "true" ability what he could play or what he formerly played, respectively.
An example for lack of motivation would be the 1st round match in Rome where it was very clearly visable and he himself said that it was a better deal for him losing first round (after playing the Munich semifinal on Saturday before) than pulling out on short notice of a Masters and probably getting fined. But overall I think the motivation to play is there in general. Otherwise he would not have fought back from his injury where he really did a lot, even during worst times as he couldn't even hold a racket, let alone striking the ball, in his right hand and as he was practising with his left hand to avoid losing touch and the feeling for game and footwork completely. Most of the times the frontier between lack of motivation and frustration is very close, same for Tommy, maybe even more than for other players. But overall I think he just can't express his motivation as well as others: he doesn't celebrate the great points as emphatically as he is scolding and quarrelling furiously with himself after playing pathetic points, so this could leave people sometimes wondering: "what the fuck? this guy is leading, has everything set up in his favour, just played a perfect point and no really positive reaction or body language?" Also, he's is hardly somebody who is running around and who is underlining in each interview or statement that he wants to win everything, that he wants to win Wimbledon... things like that.